STRUCTURE, DEVELOPMENT AND REPRODUCTION IN
FLOWERING PLANTS
Shoot System (Meristems, Stem Structure and Anatomy and Secondary
Growth)
Dr. (Ms.) Mrinal Bhargava
Reader
Department of Botany
Zakir Hussain College
Jawahar Lal Marg
New Delhi - 110001
Date of Submission: September 11, 2006
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INDEX
1. TEXT
PAGE 2
2. ILLUSTRATIONS
PAGE 32
3. ILLUSTRATION CREDITS
PAGE 55
4. SUGGESTED READINGS
PAGE 56
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SHOOT SYSTEM (MERISTEMS, STEM, STRUCTURE AND ANATOMY, AND SECONDARY
GROWTH)
The term shoot is a collective term for the stem and its appendages. The stem is divided into nodes and
internodes and one or more leaves may be present at each node. Besides leaves, buds also occur on the shoots.
Those in the axils of leaves are the axillary buds while the shoot is terminated by the apical or the terminal bud.
(Fig.1). Depending on the level of development of internodes, the shoot assumes varying heights. In majority of
the plants, internodes are elongated structures, though deviations occur, as is the case in rosette plants. In the
rosette plants, the internodes are highly condensed, resulting in the subsequent crowding of nodes and therefore
leaves as well, as seen in cabbage. Differences are also observed in the morphology and structure of the shoot
depending on the environment and function. Thus, when the shoot becomes underground, it may develop into
bulbs (onions), tubers (potato), rhizomes (ginger) or corms (yams) which serve as perennating and storage
organs. Shoots may also be fleshy and succulent, as are commonly observed in the Cacti and Euphorbias, and
carry on with photosynthesis in dry habitats.
THE SHOOT APICAL MERISTEM
The apical meristem of the shoot is organized during the development of the embryo in seed plants. It is
organized in the embryo just before or after the initiation of cotyledonary primordia. The shoot apical meristem
is the region of the primary organisation of the shoot in which the processes of growth take place and these
cannot be limited to a point. The terminal part of the shoot which is immediately above the uppermost leaf
primordium is considered as the shoot apex proper. It is comprised of dividing cells or initials which are a
source of all body cells and their immediate derivatives. The initials and their most recent derivatives together
constitute the promeristem or protomeristem. The shape of the shoot apical meristem may be dome-like,
conical, flat, elongated or even concave. The cells of this region are thin walled with dense cytoplasm and they
lack large vacuoles.
The shoot apical meristem repetitively produces leaf primordia and bud primordia, resulting in a succession of
repeated units called phytomeres. Each phytomere consists of a node with its attached leaf, the internodes
below that leaf and the bud at the base of the internodes (Fig. 2).
Vegetative Shoot Apex in Vascular Cryptogams
In the vascular cryptogams, the structure of the shoot apical meristems is very simple. It has a single large
apical initial cell called the apical cell or apical initial which appears to be the source of all cells of the shoot. It
is like an inverted pyramid (Fig. 3), with its square face directed upwards and the three to four cutting faces
pointing downwards. The apical cell divides asymmetrically in which cell plate formation occurs close to one
of the cutting surfaces. During the next division of apical cell, a similar wall formation will take place along the
adjacent cutting face. Because of these parallel divisions along the cutting faces, the derived cells are arranged
in an orderly manner. In many pteridophytes, especially the ferns, cell subjacent to the apical cell or the surface
initials are a source of most cells which further divide and form the internal tissues of the stem. Hence, these
cells form a zone of rapid divisions and function as promeristem.
Vegetative shoot apex of Gymnosperms
Apical meristem in the seed plants is made up of a group of cells. The outermost layer of the shoot apex of
gymnosperms shows both anticlinal and periclinal divisions. Hence, this top layer which is also known as the
surface meristem (Fig. 4) represents the initiation zone of the entire apex. Periclinal divisions are more frequent
in the cells located in the median position of this surface layer. These median cells are the apical initials. From
these apical initials are derived a group of cells that are situated just below the surface layer, the central mother
cell zone. Cells of this zone are large, polygonal with thick walls and they are conspicuously vacuolated, a
feature generally associated with low mitotic activity. Other parts that are distinctly observed in the shoot apex
are the peripheral (or the flank) meristem which is present on the peripheral region and the rib (or the file)
meristem occurring right below the central mother cells. The peripheral meristem is formed from the apical
initials as well as the central mother cells and this meristem undergoes active mitotic activity. The pith
meristem arises from divisions along the periphery of transitional zone. Increase in the height/length as well as
the width of the stem is a result of anticlinal and periclinal divisions respectively.
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Vegetative shoot apex in Angiosperms
In the angiosperms as well as in the gymnosperms Ephedra and Araucaria, the tunica - corpus concept proposed
by Schmidt (1924) is the most widely accepted one. According to this, the apical meristem can be distinguished
into two zones on the basis of planes of cell divisions which occur in them. Hence, there is an outer tunica
consisting of one or more layers and an inner corpus (Fig. 5). Cells of tunica divide only anticlinally while the
cells of corpus divide in different planes. Thus tunica increases in surface area and contributes to surface
growth, while the cells of corpus increase the volume. The number of layers in tunica varies from species to
species and also within a species. Most commonly, a single layered or monostratose tunica is observed.
However, 2-5 layered or multistratose tunica is not uncommon. In Xanthorrhoea media, as many as 18 layers of
tunica have been reported (Staff, 1968).
Both corpus and tunica have their own initials. Cells in the median part of the apex are initials for the tunica.
When these divide, some of the new cells remain as initials while the remaining adds to the derivatives. These
derivatives further divide and contribute to the surface of the shoot. Initials, for the corpus occur right below the
tunica. These divide periclinally, forming derivatives for the corpus below. These derivatives produce cells
which are added to the pith meristem as well as to the rib and peripheral meristems in the shoot tip.
Sometimes, especially in the monocotyledons, where the tunica is multistratose, periclinal divisions occur in the
inner layers while the outer layers divide only anticlinally. Thus, to differentiate the two groups of the apex,
Popham & Chan (1950) use the terms mantle and core for all the outer layers and the inner group of cells
respectively. The term tunica is reserved strictly for the outer layers showing only anticlinal divisions whereas
mantle includes all the layers dividing anticlinally as well as the cells of inner tunica layers towards the corpus
showing periclinal divisions. The cells included in the core divide in all planes. The surface layers of the shoot
meristem may be also designated as L1, L2 and L3 (Fig. 6). L1 and L2 usually divide anticlinally, whereas L3
divides both anticlinally and periclinally.
Central part of the shoot apical meristem is the central zone and it is the region of low mitotic activity. It
contains usually few cells that divide slowly and give rise to cells below and on the sides. Cells formed below
constitute the rib meristem which generates the central tissues of the stem and contribute in the internode
elongation while cells formed on the side constitute the peripheral zone. The cells in the latter divide rapidly
and lateral organs like leaves are initiated here.
The tunica - corpus theory differs from the Histogen theory of Hanstein (1868) according to which the apical
meristem of the shoot has distinct group of cells that give rise to three independent histogens - dermatogen,
periblem and plerome. According to the theory, these three histogens - dermatogen, periblem and plerome are
predestined to form the epidermis, the cortex and the stele respectively. A similar view has been put forth by
Gahan (1981) and Mauseth (1982). The shoot apical meristem is considered by them as a collection of several
distinct meristems that function together to establish the pattern of shoot being formed. These authors base their
concept on facts that (1) all cells have equal potential, and (2) as cells flow from the central mother cell zone
into the peripheral region or the pith - rib meristem, their metabolism is adjusted as indicated by changes in size,
shape and ultrastructure. They have suggested that part of the reason that the cells of different zones like cortex,
pith have unique metabolisms is that the differentiation of their derivative cells has already begun even while the
cells are still in the apical zones (Mauseth).
The shape of the shoot apical meristem shows periodic changes with the initiation of leaf primordia, i.e. it
undergoes plastochronic changes (plastochron is the time difference between the initiation of one leaf
primordium and the next). With the initiation of a new leaf primordium (initially referred to as a leaf buttress,
until the foliar primordium becomes distinguishable), the width of the apical meristem is maximum and it is said
to be in the maximum phase of development. Once the leaf primordium differentiates, the meristem becomes
small and narrow again and it is now said to be in the minimal phase of development. Hence the shoot apex
shows a regular rhythmic maximal and minimal phase of development (Fig. 7). In this aspect, it differs from the
root apex, where no such changes occur, since the lateral roots do not originate at the root tip (the root tip also
differs from the shoot tip in being sub - apical in position as it is covered by a root cap).
Thus, it is seen that in the shoot tip, below the apical meristem, is the region of organogenesis (e.g. initiation of
leaf primordia) and histogenesis. Regions or meristematic precursors from which the epidermis, cortex and
vascular tissues are to originate are identifiable. These regions are - protoderm, ground meristem and
procambium. The procambium cells are narrow and elongated and form the precursor stages of the vascular
tissues, while the cells of the ground meristem are distinct as they are wider than the procambial cells and also
have more vacuoles. Protoderm is the outer most layer, and its cells divide anticlinally giving rise to the
epidermal layer. The stem tip cannot be divided into regions of cell division, elongation and maturation as is the
case in the root tips, since meristematic and differentiation activities overlap. During active growth, leaf
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primordia are formed rapidly and nodes and internodes are not distinguishable. As growth begins between the
levels of leaf primordia, nodes and internodes become discernable.
Internodal elongation
The shoot, as we know, is divided into nodes and internodes. The site, at which a leaf is initiated, is designated
as a node, while the intervening segment between two adjacent nodes is internode. When actively growing, the
shoot apex gives rise to leaf primordia in such a rapid succession that nodes and internodes cannot be
distinguished at first. Later, with growth occurring between levels of leaf attachment, i.e. nodes, the elongated
parts of the stem, i.e. internodes become recognizable. Growth takes place by prominent transverse divisions in
the cells in the ground tissue above each node, resulting in longitudinal files or ribs of cells, which as mentioned
earlier, are referred to as the rib meristem. Cells derived by this activity elongate, resulting in the growth of
internodes. This elongation mostly occurs simultaneously over several internodes and as a result of this
elongation, the shoot apex and the young leaf primordia are carried upwards. Most of the increase in length of
internodes is because of cell elongation, though cell divisions which take place also contribute.
The meristematic activity causing elongation of internodes may be fairly uniform throughout the internodes, or
it may occur as a wave progressing from the base of the internodes upward. In many plants, Graminaceous
members for instance, a localised meristematic region - intercalary meristem (a meristematic tissue intercalated
between two regions that are more advanced) at the base of the internodes is largely responsible for the growth
of the internodes. This meristem may retain the ability to divide long after the elongation of internodes takes
place.
Branching patterns in the stem
Stems are repeatedly branched in order to expose all leaves to uniform light. Branching is also useful in
spreading the whole weight of foliage evenly on all branches so that the tree attains mechanical stability.
Branching is mainly of two types - dichotomous and lateral. Lateral branches develop from axillary buds in the
axils of leaves and as the name suggests they are lateral in position. In a large number of flowering plants, the
apical meristem of the terminal bud retains the ability to divide indefinitely. During its course of development,
it produces lateral branches in an acropetal succession. Such a main axis of continuous terminal growth is called
a monopodium and this type of branching is also known as monopodial branching or indefinite branching.
Lateral branches arising from axillary buds spread out horizontally. The older branches at the bottom are long
and the young ones towards the apex are progressively shorter. As a result, the entire tree may appear conical or
pyramidal in shape, as seen in Casuarina, Polyalthia, conifers. However, in many other plants, the apical
meristem of the terminal bud after continuing to contribute in the elongation of the stem may get modified to
form a reproductive axis or a tendril or a thorn or some other structure. At such a time, one of the uppermost
true axillary buds becomes active and replaces the terminal bud and contributes in forming the shoot system.
The lateral axillary bud may cease to be active too after forming some leaves. This type of branching is termed
as sympodial or definite type of branching. It is not really a single structure but a series of branches on
branches. Hence, in this type, the growth of the axis is limited.
Dichotomous branching is rare in the seed plants, though it is of common occurrence in the cryptogams like
Riccia, Marchantia and Lycopodium phlegmaria. Amongst the Angiosperms, it has been confirmed only in a
few genera like Hyphaene, Nypa fruticans, Chamaedorea, Flagellaria indica, Asclepias syriaca, Echinocerius
reichenbachii and Mamillaria sp. In vascular cryptogams, branching results from a dichotomous division of the
apical cell, followed by growth taking place in order to form two separate branches.
PRIMARY STRUCTURE OF STEM
As has been mentioned earlier, the primary stem consists of the dermal, ground and the vascular tissues. The
arrangement and structure of primary tissues is discussed below.
Epidermis – The outermost layer of the primary stem is the epidermis. It is commonly composed of a single
layer of closely arranged cells (Fig. 8). The outer walls may be covered by cuticle. Since the epidermal cells
are living, they are capable of dividing. This is important as the epidermis is subjected to stresses during
increase in girth of the stem during primary and secondary growth. Divisions, followed by enlargement allow
the epidermal layer cope with this increase. Stomata may occur in the stem epidermis, especially in the
herbaceous plants, though they are less prominent here as compared with the leaves. Another structure
commonly associated with the epidermis are the trichomes. Trichomes of various types occur here and many
are characteristic of certain taxa. Epidermis persists throughout in the stems of many herbaceous plants as well
as in most of the monocots. However, as secondary growth progresses in the shrubby and arboreal dicots,
epidermis is sloughed off with the formation of periderm. In many monocots too, a special periderm or storied
cork replaces the epidermis.
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Cortex – is the region between the epidermis and vascular cylinder (Fig. 10). It is comprised of different types
of cells. In many plants, it may be made up of parenchyma cells only. However, in a large number of flowering
plants, the outermost layers of the cortex are collenchymatous (Fig. 9) or sclerenchymatous in nature and form
the hypodermis. These hypodermal tissues may form a continuous layer (Helianthus) or they may occur in the
form of strands (Cucurbita). Sclerenchyma is more common in the grasses. Both help in providing support to
the stem. Chlorenchyma cells are also commonly found in the stem cortex, thus making the stem assimilatory in
nature. Intercellular spaces are present in between the parenchyma cells. In stem cortex of aquatic
angiosperms, aerenchyma is well developed, forming a system of large intercellular spaces which provide
mechanical strength to withstand the stresses caused by water currents. Cortical tissues may also contain
different types of idioblasts, including sclereids or secretory cells. Cells containing various kinds of ergastic
substances or crystals are common. Laticifers, resin ducts or other special structures may also occur in the cortex
region. The innermost layer of the cortex is the endodermis. Though the endodermis is morphologically easily
distinguishable in the vascular cryptogams, it is not so in the Gymnosperms and Angiosperms. In the young
dicot stems, the endodermis is recognisable as the starch sheath as it contains abundant starch. Casparian strips
may be observed in some dicot stems in the endodermal cells. Whether a typical endodermis (a layer of barrel
shaped cells devoid of intercellular spaces) is morphologically identifiable or not, a physiological boundary
demarcates the vascular tissues from the cortex in the stem.
The importance of endodermis in the few stems that it has been observed in is that it channelises the water from
the stele and prevents its entry into the cortex. The casparian strips prevent the movement of water
apoplastically. Endodermis in the stem may be found in root- like stems such as rhizomes. e.g. Acorus.
Pericycle – is clearly visible in the roots of all vascular plants as well as in the stems of the lower vascular
plants between the endodermis and the vascular tissues. However, in the seed plants, it is absent in most stems.
In several stems, perivascular fibers, alternatively also known as pericyclic fibers are located along the outer
periphery of vascular cylinder. These originate from the tissue between the endodermis and phloem. e.g.
Cucurbita, Aristolochia
Pith – is the region in the central part of the stem surrounded by the vascular tissues (Fig. 10). It was earlier
known as the medulla, a term that is not in use now. The tissue, generally parenchyma, connecting the pith with
the cortex is known as the medullary rays. In most plants the pith is composed of parenchyma tissue with welldeveloped intercellular spaces. Rarely, the pith may comprise of thick walled, liguified parenchyma cells.
Idioblasts, in the form of laticifers, secretory cells, sclereids or libers or cells with ergastic substances are
common. The peripheral part of the pith may appear distinct from the inner part as it may be composed of small
thick walled, compactly arranged cells. This peripheral zone is referred to as the medullary sheath or the
perimedullary zone.
During the development of the stem, the growth of the pith cells in the internodes of many species stops as they
mature early, while the surrounding tissues continue to divide and enlarge. This leads to the pith cells being torn
apart and the region then becomes hollow, especially in the internodes regions. The nodes retain their pith,
which is often termed as the nodal diaphragm. In the stems of some plants like Phytolacca americana, and
Juglans, the pith becomes chambered by the formation of parenchymatous diaphragms in the hollow
internodes.
Vascular Tissues – This occurs in the form of discrete organized strands (Figs. 8; 10). Each of these is referred
to as a vascular bundle and is composed of primary xylem and primary phloem. The size, number and
arrangement of these strands vary amongst the seed plants. The primary phloem is present usually externally,
while primary xylem is usually located internally. Such a vascular bundle in which the phloem occurs on one
side of xylem is known as a collateral bundle and it is of the most common occurrence. In certain
Angiospermous families, besides the external phloem, internal phloem may also be present in the vascular
bundles.
Such bundles are termed as bicollateral. e.g. Cucurbitaceae, Asclepiadaceae, Apocyanaceae,
Convolvalaceae as well as certain tribes of Asteraceae. Vascular bundles may also be concentric in nature
which may be either amphicribral or amphivasal in nature. In the former, phloem surrounds the xylem and in
the latter, it is surrounded by xylem. Amphicribral vascular bundles are more common in the ferns, e.g.
Polypodium while amphivasal bundles are commonly formed in several monocots undergoing secondary
growth. Cordyline, Aloe arborescens, Dracaena, Acorus, Convallaria majalis and some genera of
Xanthorrhoeaceae have amphivasal vascular bundles. Depending on the presence or absence of meristematic
cells or intrafascicular cambium in the vascular bundles, they are termed as open or closed vascular bundles.
Dicotyledonous stems are characterised by open bundles whereas those in monocots are of the closed type. (Fig.
11) Within the xylem of any vascular bundle, a gradation in the diameter of tracheary elements occurs. Those
elements with the smallest diameter occur towards the interior of the stem and are termed as the protoxylem, and
on the outer side are elements of a larger size, known as the metaxylem. Hence in the stem, the protoxylem has
an endarch position (in contrast to the root, where it is exarch in position) (Fig. 8). As the name indicates,
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protoxylem elements are the first to differentiate in the region below the shoot apical meristem. They have
tracheary elements that are short in size and narrow in diameter because they have little time to grow.
Metaxylem on the other hand, differentiates later. The protoxylem elements have either annular or helical
thickenings on their longitudinal walls (Fig. 12). These thickenings allow the elements of protoxylem to be
pulled into a longer shape while the adjacent cells are still enlarging. The metaxylem elements differentiate
after the stem expansion stops and therefore, they may develop other types of secondary wall thickenings. Very
often, much of the protoxylem elements get damaged during expansion and lacunae are formed (Fig. 13).
Because of this, metaxylem forms the main pipeline for conduction in the primary plant body and in the
monocots, it functions for a long time. A similar situation is seen in the primary phloem. The early
differentiating phloem elements are small and form protophloem and the later formed ones are larger and
comprise the metaphloem. The sieve elements have distinct sieve areas in the metaphloem and they function for
a much longer period like the metaxylem.
Stele – The term stele means the central column of the stem and root. It includes all the vascular tissues, pith (if
present), pericycle and interfascicular regions (Van Tieghem)
Different kinds of steles are present in the vascular plants. The simplest and also phylogenetically the most
primitive stele is the protostele which has vascular tissues in the form of a solid column. Protostele may be in
the form of a haplostele, plectostele, actinostele or mixed protostele as is depicted in figure 14. These are
commonly observed in certain Pteridophytes, like members of Lycophyta and Sphenophyta, as well as in the
stems of certain aquatic Angiosperms.
Siphonostele is the next type of stele. It is tubular in nature with the central part of the column being non
vascular in nature. Therefore, a cylinder of pith is present in the middle of the stem. Two variations are present
in the siphonostele – ectophloic and amphiphloic (Fig. 14), siphonostele depending on whether phloem occurs
on the outside of xylem or on both the sides. This type of stele is found in vascular cryptogams belonging to
Lycophyta, Sphenophyta and Pteridophytes. Endodermis forms a border just outside the phloem area in
siphonostele. A simple siphonostele is a continuous cylinder with no leaf gaps (Fig. 15). It may also be made
up of a network of bundles. The vascular tissues in a cross section of this type of siphonostele appear as discrete
bundles, with regions between them being parenchymatous. Parenchymatous regions formed in the stele above
the positions where leaf traces depart from the stele to leaves are termed as leaf gaps (Fig. 15). A stele with leaf
gaps is known as solenostele and an amphiphloic siphonostele with many leaf gaps at different levels is a
dictyostele in which each bundle is a meristele. Eustele, with collateral vascular bundles has developed from
ectophloic siphonostele. In bicollateral vascular bundles, internal phloem is a result of secondary specialisation.
Vascular bundles are interconnected in a dictyostele, as well as in eustele (Fig. 15). The type of stele in which
vascular bundles are scattered is termed as an atactostele, a common feature of monocot stems.
Anatomy of the Node
Nodes are complex anatomically because it is here that the vascular tissues of the lateral appendages like leaves
and branches are connected with those of the stem. These vascular strands or bundles in the stem that extend
between the connection with a leaf and that of the stem are called leaf traces. Where the leaf trace departs from
the vascular cylinder in the stem, a parenchymatous region occurs which is termed as a leaf gap or lacuna.
Number of leaf traces and leaf gaps vary in the different taxa and also at times within an individual plant at
different levels. The leaf traces may traverse one or several nodes and internodes before entering a leaf.
According to the number of leaf gaps per leaf, the node may be unilacunar, trilacunar or multilacunar (Figs. 16;
17). The number of leaf traces associated with the gaps may vary. The study of nodal anatomy has been
extensively used for taxonomy and phylogeny. Sinnott (1914) postulated that a trilacunar node is the most
primitive node and that the unilacunar node arose phylogenetically either by the elimination of the two lateral
gaps (and their traces) or by their approximation with the median gap. According to him, the multilacunar node
arose from the trilacunar node by the formation of additional leaf gaps and traces (Fig. 18). However, Marsden
and Bailey (1955) considered the unilacunar – two trace node to be the most primitive as it is found in many
ferns, Ginkgo biloba, seed ferns, Cordaitales, Bennettitales, Coniferales, Ephedra, as well as in several Ranales
members. According to them, the trilacunar and multilacunar nodes arose from the unilacunar node (Fig. 16).
Unilacunar 2-trace node also occurs in many advanced dicotyledonous taxa belonging to Laurales, Verbenaceae,
Labiatae and Solanaceae. Many other views and interpretations put forth by Botanists are not being included in
this discussion. The views of Takhtajan (1964) regarding the possible course of evolution of nodal structure are
shown in Figure 19. He considered the trilacunar node, with the median gap associated with two traces as the
most primitive. The other nodal types according to him have arisen from this type.
Secondary Growth in Stems
In contrast to the primary growth which takes place in the apical meristem, secondary growth results from the
activity of lateral meristems - the vascular cambium and the cork cambium. In most monocots and some
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herbaceous dicots like Ranunculus sceleratus, growth of the plant body ceases with maturation of primary
tissues. However, in the Gymnosperms, as well as in a majority of dicots, increase in diameter or girth takes
place with the help of these meristems. Secondary growth is generally lacking in the monocot stems. However,
it occurs in a few genera and is of a special type.
The structure and activity of the vascular cambium will be discussed before discussing the secondary growth in
the stem.
Vascular Cambium – Structure and Activity
Vascular cambium is a meristematic layer that is formed by the joining of the fascicular and interfascicular
cambia. It appears like a continuous ring in most plants, or as a cylinder in the stem. However, in many
herbaceous genera, succulent Euphorbias and Cacti, the vascular cambium exists as narrow strips in the vascular
bundles and produces little wood. Open vascular bundles are present in the dicot stems. Cambial cells present
here between the primary xylem and primary phloem is formed from the procambium. Hence, fascicular
cambium arises from the procambium while the interfascicular cambium is differentiated from the parenchyma
cells between the vascular bundles (Fig. 8; 20). All these join together to form a complete cambium ring. As
the cells of this layer divide, they produce layers of secondary xylem to the inside and secondary phloem
towards outside (Fig. 21). These meristematic cells are highly vacuolated, unlike the dividing cells of apical
meristems which are densely cytoplasmic with large nuclei. The vascular cambium consists of two types of
initials – (1) fusiform initials, and (2) ray initials (Fig. 22). The fusiform initials are highly elongated cells, their
length being several times their width, and they have tapering ends. They are given the name fusiform initials
because they appear to be spindle shaped in a tangential longitudinal section. The length of the fusiform initials
varies in the various dicots and Gymnosperms. It is much more in the Gymnosperms, varying from 700 µm to
8700µm (in Sequoia simpervirons) while in the dicots, it varies from 140 -462 µm. The length of fusiform
initials in closely related species may be quite different and in a species or even an individual it varies during the
different seasons in the year. The longitudinal walls of these cells appear to be beaded due to the presence of
many pits. The ray initials on the other hand, are smaller in size and are almost is odiametric in shape. The
fusiform initials divide repeatedly along their longitudinal axis and contribute in the formation of those
components of secondary xylem and secondary phloem that are aligned along the axis of the stem. Hence, the
derivatives of fusiform initials that are formed centripetally differentiate into fibres, tracheary elements –
tracheids and vessel members and axial xylem parenchyma. Centrifugally formed derivatives develop into sieve
elements (sieve cells, sieve tubes), bast fibers, albuminous cells, companion cells and axial phloem parenchyma.
The ray initials divide and contribute towards the formation of ray xylem parenchyma and ray phloem
parenchyma, both of which are present along the radial axis of the stem (Fig. 23) Depending upon the
arrangement of the fusiform initials, vascular cambium is of two types – 1) Storied or stratified cambium and 2)
Non - storied or non - stratified cambium. In the storied cambium, the fusiform initials are arranged in
horizontal rows or in regular tiers, with their end plates at the same level. Also fusiform initials in the storied
cambium are relatively shorter in length than those occurring in the non- storied cambium. Their length here
may vary from 140µm to 520µm. In the non-storied cambium, the fusiform initials do not have a regular
arrangement (Fig. 22). Also, these initials are relatively longer with their length varying between 230µm and
2300µm. In the vessel less Angiosperms it may be up to 6300µ. In Gymnosperms the length of fusiform initials
lies between 100µ and 8700µ. The ends of these initials overlap each other strongly. Non storied cambium
occurs in a larger number of genera as compared with storied cambium. Storied cambium is found in Diospyros
virginiana, Aesculus, Cryptocarya, Ficus, Tilia and many members of Fabales.
Storied cambium is considered to be phylogenetically advanced and is believed to have evolved by the gradual
reduction in the length of fusiform initials. The type of vascular cambium determines the type of wood that is
formed in the stems. Storied wood is formed from storied cambium while non- storied wood is formed from
non- storied cambium respectively
Divisions in the vascular cambium:
Cambium layer becomes inactive or dormant during stress conditions. Stress may be imposed on this layer by
heat, cold or lack of water. At such times, the cambial zone appears narrow as the cells enter dormancy and cell
division stop, and many of the xylem mother cells and phloem mother cells mature. Many times at least some of
the partially differentiated xylem mother cells and phloem mother cells become quiescent. Once the cause of
stress is over or when conditions become favourable, these quickly complete differentiation and the plant has
new conducting tissues early. After overwintering, in the spring as the cambium resume cell division, the first
cells to become reactivated are those immediately below the buds. Cell division spreads downwards, perhaps
being triggered by the basipetal movement of the auxin. The cambial zone appears rather wide during the
summer season when the temperatures are favourable. Fusiform initials divide rapidly to form xylem mother
cells and phloem mother cells which in turn divide rapidly to form components of secondary xylem and
8
secondary phloem respectively. The ratios of division are far greater than the rates of differentiation resulting in
a broad cambial zone . In regions where temperatures are favourable throughout the year, cambial activity
occurs during the whole year.
Fusiform initials divide periclinally longitudinally to form new derivatives for secondary vascular tissues. Such
divisions are known as additive or proliferative divisions. Of the derivatives produced by such divisions a large
number of cells differentiate into xylem mother cells and fewer become phloem mother cells. As the xylem
matures interior to the vascular cambium, the latter expands. The cambium is pushed outward, and its
circumference constantly increases. In order to keep up with this increase the cambial cells divide to produce
more initials. Such divisions which increase the number of cambial initials are termed as multiplicative
divisions. Multiplicative divisions in the fusiform initials of storied cambium are true anticlinal (Fig. 24), where
the cell plate is formed perfectly from tip to tip, resulting into two daughter cells which are of equal length and
the ends of the cells are aligned with the position of the original mother cell wall. Due to this kind of division,
tiers of the fusiform initials are maintained in regular rows. Multiplicative divisions in the short fusiform initials
may also be radial anticlinal or by lateral anticlinal walls. (Fig. 25). In the nonstoried cambium, multiplicative
division occurs by pseudotranverse division especially in the very long fusiform initials (Fig. 24) resulting in
two daughter cells that are shorter than the mother cells. The original length is attained by the daughter cells by
the occurrence of intrusive growth (Fig. 25). As seen in the preceding paragraphs the fusiform initials during
cell divisions, form cross wall along the maximal area of the cell. In this respect, they disobey Ererra’s law,
according to which cell plate formation in a dividing cell is formed along the minimal area of the cell.
Vascular Cambium in Monocots: Special type of vascular cambium occurs in some woody and herbaceous
Liliflorae taxa like Dracaena , Agave, Aloe, Yucca, Cordyline and Sansiviera. This meristematic layer is present
outside the primary vascular bundles in the outer ground tissue of the stem and it’s activity differs from that of
the vascular cambium of dicots, resulting in anomalous secondary growth (discussed later under anomalous
secondary growth). The cells of the meristematic layer may vary in shape and may be fusiform or polygonal or
rectangular in shape. Variations in these cells may be observed within the same plant.
Secondary growth due to Vascular Cambium
A ring of vascular cambium is formed in the stem. It arises partly from the procambium that remains
undifferentiated between the primary xylem and the primary phloem (also called fascicular cambium) and
partly from the parenchyma cells between the fascicular or vascular bundles (also called inter-fascicular
cambium) (Fig. 20). The initiation of divisions in the vascular cambium takes place after the maturation of
primary xylem and the primary phloem. Frequently divisions in the fascicular cambium precede those occurring
in the inter fascicular region. The cells of the vascular cambium ring divide periclinally, forming derivatives on
the inner side of the cambium ring as well as on the outer side. The former derivatives differentiate and mature
into secondary xylem, while the latter give rise to components of secondary phloem. In most dicotyledonous and
the Gymnospermous stems, the amount of secondary xylem formed is much more than the secondary phloem.
As a result, after the occurrence of secondary growth, bulk of the stem is made up of the secondary xylem or
wood (Fig. 20). Medullary rays are formed by the ray initials of the cambium in the secondary xylem and the
secondary phloem. These rays are made up of parenchyma cells cut off by the ray initials.
With progressive increase in the secondary growth, more and more secondary xylem and the secondary phloem
are produced. With an increase in the secondary vascular tissues, pressure is exerted on both the primary xylem
and the primary phloem. The primary phloem gets pushed outwards, becomes non- conducting and is eventually
completely crushed. The primary xylem elements do not get distorted, but these may get pushed into the pith
region.
SECONDARY XYLEM AND VARIATIONS IN WOOD STRUCTURE
Secondary xylem is formed by the activity of the vascular cambium which deposits cells towards its interior side
that get differentiated into secondary xylem. This secondary xylem is wood. Woods are commonly classified
into hardwoods and softwoods. The so called hardwoods are Angiospermous woods, while the softwoods are
produced by Gymnosperms, mainly conifers. These two woods have basic structural differences, but the terms
“hardwood” and “softwood” do not accurately express the relative density or hardness of the wood. For instance
one of the lightest and also the softest woods is balsa wood (Ochroma lagopus) which is an Angiosperm. Many
conifers, for example certain species of Pinus produce woods that are harder than some hardwoods produced by
Angiosperms. Gymnospermous woods are homogeneous in nature, being composed chiefly by tracheids. These
tracheids are aligned in a long, straight manner making the wood especially suitable for paper making. The
Angiosperm woods on the other hand are heterogeneous in nature and they have various components like
tracheids, vessel members, libriform fibres, fibre-tracheids and parenchyma. Wood of certain Angiospermous
9
genera like Acacia, Carya Eucalyptus, Quercus are especially dense and strong because of the high percentage
of fibres in them. Some dicot woods however are as simple as those of conifers and these are the primitively
vesselless genera. These belong to five families- Chloranthaceae, Winteraceae, Tetracentraceae,
Trochodendraceae and Monimiaceae and examples of a few genera are Trochodendron , Tetracentron, Drimys,
and Pseudowintera.
Secondary Xylem of Gymnosperms: The secondary xylem of Gymnosperms (Fig. 26) is relatively simple in
structure, as it is chiefly made up of imperforate tracheids. Fibre tracheids may occur in the late wood, but
libriform fibres are absent. Since there are mostly just tracheids in conifer wood, these cells must function not
only to conduct and store water but also to provide mechanical support to the tree. The shape and structure of
the tracheids is suited for these functions. The tracheids are long and greater length is advantageous to both
conductivity and strength. Regarding width of the tracheids, narrower diameter is better for the strength and
greater diameter increases conductivity. These opposing demands are met by the conifers by producing two
types of tracheids in the late wood and early wood. Early wood is the secondary xylem which is formed during
spring season when abundant water is available from melting snow and rain water. The tracheids developed at
this time are quite wide and have relatively thin walls (Fig. 27).
Late wood is produced by the trees during summer and early autumn when the soil begins to dry. The tracheids
formed during this time have a narrower diameter and thicker walls (Fig. 27). At the very end of the growing
season, fibre-tracheids may be produced. Since the tracheids of spring and summer wood are distinct, clear
annual rings are visible in the coniferous wood. One of the major features of the Gymnospermous woods is the
absence of vessels (vessels are present only in the three genera of Gnetopsida) and the presence of relatively
small amounts of axial parenchyma. In Pinus the axial parenchyma is associated with resin ducts. These resin
ducts are large intercellular spaces lined by thin walled epithelial cells that secrete resins into these canals. Resin
canals occur in the parenchyma of both axial as well as radial rays in Pinus (Fig. 28) Resin duct formation can
be stimulated by factors like frost, strong winds, wounding, pressure and injuries or trauma of some other
nature. Such resin ducts are often referred to as traumatic resin ducts. Resin is protective in nature and protects
the wood from microbial, fungal and insect attack.
The tracheids of coniferous woods are characterised by the presence of large, circular, bordered pits on their
radial walls. Pits are more abundant on the end of the cells, where they overlap with other tracheids. The pitpairs (pair of pits) between conifer tracheids are each characterised by the presence of torus (plural-tori). The
torus is thickened central portion of the pit membrane, consisting mainly of middle lamella and two primary
walls. It is slightly larger than the openings or apertures in the pit borders (Fig. 29). The pit membrane is
flexible, and in certain conditions, the torus may block one of the apertures and prevent the movement of water
or gases through the pit- pair. Tori have recently been reported in the bordered pit pairs of tracheids and vessel
elements in several genera of dicots.
The transverse system of rays produced by ray initials and longitudinal or axial system produced by fusiform
initials are both present in the conifers. These two systems work together in the functioning of the wood. The
axial system provides longitudinal conduction and mechanical strength. Because of the types of cells present
and their orientation, however the axial system is not well adapted for either carbohydrates or mineral storage
and it is extremely inefficient in transverse conduction necessary to keep internal tissues in communication with
the water and the nutrient stream of the actively conducting secondary xylem and secondary phloem. This shortdistance, transverse conduction and nutrient storage are the responsibility of the rays.
Rays in the conifer wood may be homocellular or heterocellular in nature depending upon the constituent cells.
Homocellular rays consist of only parenchyma cells, whereas in heterocellular rays, ray tracheids are also
present along with parenchyma cells. Ray tracheids occur in conifer like Pinus, Tsuga, Pseudotsuga, Larix and
Picea and these can be distinguished by the presence of bordered pit in their secondary walls. The presence of
increased quantities of ray tracheids results in more rapid water movement. As mentioned earlier, rays help in
horizontal conduction and they also interact with the longitudinally oriented tracheids. For this purpose, pitting
on all the walls is very important. Where the vertical tracheids come in contact with ray parenchyma cells, the
pit-pairs are usually half-bordered, i.e. the bordered pit is situated on the side of the tracheids and the simple pit
on the side of the parenchyma cell. This area of contact between a ray parenchyma cell and single vertical
tracheids is termed as a cross-field.
SECONDARY XYLEM OF ANGIOSPERMS:
The discussion of wood in Angiosperm is restricted to the dicots because none of the monocots shows the kind
of secondary growth that is observed in the dicots. Much variation is seen in the dicot wood structure depending
on the arrangement of the component cells in secondary xylem. Also as in the case of Gymnosperms, in a large
number of dicots the vascular cambium functions episodically- it is active during favourable seasons like spring
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and becomes quiescent or dormant during the unfavourable seasons like winter. However, in many tropical
woody plants, the activity of the cambium remains almost same throughout the years.
Ring porous and diffuse porous wood
A vessel member in a transverse section appears as a pore. Depending on the arrangement of pores or vessels,
two main types of woods can be identified - ring porous wood and diffuse porous wood (Fig. 30). In ring
porous woods, vessels of a larger diameter are formed during spring season. Their number is larger and several
rings of such pores or vessel members can be observed in cross sections of such woods. Ring porous condition
is generally formed in trees growing in temperate areas like Castanea, Fraxinus, Quercus, Ulmus, Sassafras,
Maclura, Robinia, and Celtis. The pores of vessel members formed in late wood are fewer in number and they
also have a much smaller diameter. This periodicity in the rings containing alternately large and small sized
pores results in annual increments or annual rings. Ring porous wood is also found in plants growing in arid
habitats. Each annual ring here is made up of one increment each of large and small pores. In ring porous
woods, conduction of water occurs almost entirely in the outermost growth increment at a rate which is far
greater than that observed in diffuse porous woods. In the very wide springwood vessels, water movement is up
to ten times than movement in diffuse porous woods. Ring porous condition is also regarded as an indication of
evolutionary specialisation.
In diffuse porous condition vessels of more or less similar diameter are differentiated in the secondary xylem
almost throughout the year. Hence, there is no distinction in the diameter of the vessels produced during
different seasons. Diffuse porous woods are found in Juglans, Acer, Cornus, Salix, Magnolia, Populus, and
Pyrus besides others. This kind of wood is produced in regions with more uniform climate. However, although
climate influences is important for the development of ring –porous and diffuse porous woods, it is certainly not
the only factor, because trees with both these conditions grow besides each other.
Storied and non-storied wood
These two terms refer to the arrangement of the cells of secondary xylem in the tangential sections. In certain
woods, cells of one tier unevenly overlap those of another tier. In other words, they are not arranged in regular
horizontal tiers. Such woods are non-storied or non-stratified woods and are derived from non-storied cambia.
These are found in Juglans, Fraxinus, Castanea. However, there are woods like Ficus, Dioscorea and Aesculus,
in which the tangential sections reveal horizontal layers of cells. These are the storied or stratified woods formed
by the activity of storied cambium. The latter from the evolutionary point of view, are regarded as more highly
specialised woods in comparison to the non-storied woods.
Growth Rings
Growth rings (Fig. 31) are observed in woods due to the periodic activity of the cambium and as mentioned
earlier, this periodicity is a seasonally related phenomena in the temperate zone trees. A growth increment
includes both secondary xylem and secondary phloem (though the phloem increments are not always readily
discernible). If a growth layer represents one year’s growth, it is termed as an annual ring (Fig. 27) thus the age
of a given portion of an old woody stem can be estimated by counting the growth rings. Trees that exhibit
continuous cambial activity, such as those of tropical rain forests, may lack such annual rings entirely. It is,
therefore, difficult to judge the age of such trees. Trees with annual rings produce different sized vessels (ringporous wood) or tracheids (coniferous wood) during different seasons. This makes the identification of the rings
easy.
Abrupt changes in the available water and other environmental factors may be responsible for the production of
more than one growth ring in a year. Such rings are called ‘false annual rings’. Hence, while estimating the age
of the tree by counting annual rings, one should be careful that these false rings are not counted or else the
estimate will be inaccurate.
The width of individual growth rings may vary considerably from year to year as a function of various
environmental factors like light, temperature, rainfall, available soil water and duration of the growing season.
The width of a growth ring is a fairly accurate index of the rainfall for a particular year. When adequate
moisture is present, little variation occurs from year to year except for a gradual decrease in width with increase
in age of the tree. Growth rings are wide in the years with abundant rainfall and narrow under unfavourable
conditions or in years with inadequate rains.
In semi arid and arid regions, where there is very little rain and in Northern areas where low temperature during
growing season usually limits growth, the tree is a sensitive rain gauge. Study and analysis of the growth rings is
known as Dendrochronology and it is used as an important tool for the study of climatic history and
Archaeological dating. Dendrochronologists have developed the cross-dating technique in order to avoid
misinterpretations and to verify the identification of the false and missing rings under exceptionally
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unfavourable conditions. In some years, growth ring may be absent and these are the missing rings. Cross-dating
technique is based on the principle that more or less similar ring patterns will be produced in plants influenced
by common fluctuating environmental factors. This technique is carried out by matching growth ring series of
the living and dead trees and in this way a continuous series of rings dating back to a few thousand years has
been built up. The wood of the oldest known living specimen of bristlecone pine – Pinus aristata, believed to be
5600 years old, has helped the Dendrochronologists to find out about the temperature and moisture conditions in
the past.
Heartwood and Sapwood
As secondary xylem becomes older, it gradually becomes non-functional in conduction and storage. Hence, the
wood cells have only a limited period in which they live and carry out functions other than support. In stressful
environments, the conducting elements may become non-functional by the end of first year; however under
favourable climatic conditions, the tracheary elements may continue to transport water for several years. The
conduction of water and minerals therefore, is a function of the more recently formed outer part of the wood
which is known as the sapwood or alburnum (Fig. 31). Sapwood contains living and at least the outer layers of
this region are active in water transportation while the inner layers are for storage. The non-conducting wood
can be easily distinguished since in most genera, it is darker in appearance than the sapwood and it is termed as
the heartwood or duramen. The earlier formed secondary xylem elements in most plants gradually cease to
function and all cells mainly parenchyma in it die. The protoplasts of these cells disintegrate, there is a loss of
reserve food materials like starch and the development or infiltration of ergastic substances like gums, oils,
resins and tannins occurs. Gradual oxidation and polymerisation of phenols occurs leading to the development
of coloured substances in the heartwood. Cell walls of xylem parenchyma cells become heavily lignified.
Heartwood formation is believed to be a process that enables the plant to remove from regions of growth
secondary metabolites that may be inhibitory or even toxic to living cells. The accumulation of these substances
in the heartwood results in the death of the living cells of the wood. In Gymnosperms, the pit membranes
become rigid and fixed such that the torus closes the pit aperture. This disables the pit to conduct water and it is
said to be aspirated. The tori which consisted of mainly pectins, celluloses and hemicelluloses, become lignified.
In the Angiosperms, the vessel members become blocked by the formation of tylosis and since the lumen gets
obstructed no conduction can occur. Due to the occurrence of all the above mentioned changes, the heartwood
becomes stronger, drier and consequently more resistant to decay.
The proportion of sapwood to heartwood and the degree of visible difference between them vary greatly
between different species. Broad sapwoods can be observed in trees like Acer and Betula while thin sapwood is
present in Robinia. Similarly, colour distinction between the two also differs markedly amongst different trees.
In Pinus roxburghi, Dalbergia sissoo and Albizzia lebbeck, the colour distinction between the heartwood and
sapwood is sharp, in species like Shorea robusta it is gradual; while Populus, Abies, Salix and Holoptelea show
no clear distinction between sapwood and heartwood. Every year some sapwood (which is adjacent to the
heartwood) is converted to heartwood. Hence, every year the diameter of the heartwood increases at any given
level in the stem (or root). The sapwood does not become narrower because new secondary xylem rings are
added to it by the vascular cambium. The number of growth increments retaining sapwood characteristics varies
with species. Heartwood may sometimes be formed as a result of pathological conditions.
Wood Parenchyma
Dicot woods show the presence of both axial and ray parenchyma formed from the derivatives of fusiform and
ray initial respectively. Axial wood parenchyma amount varies in the different species of dicots. Most dicot
woods unlike Gymnospermous woods typically have axial parenchyma. Only a small number of genera have
wood that lacks axial parenchyma (Elaeocarpus, Bursera, Sonneratia, Berberidaceae) or have a very small
amount. Absence of axial parenchyma in dicot wood is regarded as a primitive character. Both amount and
distribution of axial parenchyma among other elements of secondary xylem are important considerations. There
are two main types of axial parenchyma based on their proximity with the vessel members. If they are present
immediately adjacent to the vessels, the parenchyma cells are termed as the paratracheal parenchyma (Fig. 32);
when the parenchyma cells occur independently of the vessel members and are scattered among fibres and
tracheids they constitute the apotracheal parenchyma (Fig. 32). In the latter, the parenchyma have no constant
association with the vessels, though there may be a contact between the two here and there. Both these types of
axial wood parenchyma show various modifications, as shown in figure 32.
The axial parenchymas are capable of performing functions of accumulation, storage and release of stored
reserve materials. They also constitute an important reservoir for carbohydrates, nitrogen and other metabolites.
The arrangement of the parenchyma becomes very important to fulfill a function of storage. The paratracheal
parenchyma shows functional differences from the parenchyma that is scattered among the fibres. Upon the
return of favourable conditions in spring season, stored carbohydrates like starch are mobilised earlier in the
12
paratracheal cells than in the diffuse and scattered parenchyma. These cells also show high phosphates activity
and release sugars into the vessels for rapid transport to the buds and help in the movement of water into the
vessels which get filled with gases during unfavourable periods. Parenchyma cells having distinct functional
relation with the vessels are termed as contact cells and are analogous to the companion cells of phloem.
Confluent paratracheal parenchyma (Fig. 32) is regarded as most efficient in the rapid conduction of water into
the stressed vessels upon the return of favourable seasonal conditions.
Rays of Dicot Wood
Dicot woods reveal the presence of simpler rays than the Gymnospermous woods as they are made up of only
parenchyma cells. Ray tracheids are absent though radially aligned vessels may occur in certain species. Dicot
rays may be uniseriate to being as many as fifty cells wide. The rays may be formed of either one type of cells
or of two types of parenchyma cells. In the former, the ray cells are elongated in the radial direction, such cells
are procumbent and the ray is homocellular or homogenous. In the second type of ray, besides the procumbent
cells, upright cells that the vertically elongated are also present (Fig. 33) and such rays are heterocellular or
heterogeneous rays. The ray system of a wood may consist of either homocellular or heterocellular rays or a
combination of the two types of rays. It is rare to find just a single type of ray in the dicots as ray heterogeneity
is the rule. The ray cells may contain different types of secretions, idioblasts, crystals and ergastic substances.
The ray parenchyma, like the axial parenchyma, function to help in the radial transport. Those ray cells that are
connected with the tracheary elements through pit connections function as contact cells in a manner similar to
the contact cells of axial parenchyma. Contact ray cells may be upright or procumbent in nature and they always
show prominent pit connections with the vessel members. Carbohydrates like starch are stored in the ray cells of
late wood having no contacts with the vessels. This starch is mobilised during early spring with the
environmental conditions becoming favourable. There is a significant increase in the amount of sugars in the
vessel sap in spring. In all likelihood, the newly opening buds receive a considerable portion of their
carbohydrates through the xylem.
Tyloses
In a large number of species, parenchyma - cells both axial and ray parenchyma present adjacent to the vessels
form tyloses. These are balloon-like protuberances formed from the parenchyma cells into the lumen of the
vessel members through pit cavities in the vessel wall (Fig. 34). For the formation of tyloses, the pit should be
adequately wide, at least 10 µm; if it is any smaller, the friction caused by pushing the primary wall and
protoplast of parenchyma cell is too great and leads to gummosis. Gums that are formed flow into the vessel
lumen and fill it. These outgrowths are formed when the vessel becomes inactive. They are also formed when
there is an injury or as a response to pathological conditions caused by diseases. The parenchyma cell pushes a
part of its wall and cytoplasm into the vessel lumen. Enough material may be pressed through the pit by the
parenchyma cell so that a large tylosis is formed in the vessel member, but more commonly, many tyloses are
formed in the vessel lumen, thus blocking the passage completely. Hence no conduction is possible and where
tyloses are formed prematurely in response to infection, it serves a defence mechanism and inhibits the spread of
pathogen throughout the plant.
Tyloses store ergastic substances in many woods. They may also develop secondary walls or in many cases,
even differentiate into sclereids. The presence of tyloses blocks the conduction in the vessels of woods and thus
reduces the permeability of the wood. Lower permeability of the wood is an important criterion in the treatment
of wood with chemicals and preservatives and its use in trade. During heartwood formation, from sapwood, one
of the important changes observed is the development of tyloses. They may be formed in the vessel of primary
xylem and sapwood as well under special conditions like wounding or infection.
In the Gymnospermous woods, resin ducts often become blocked by enlargement of epithelial cells; such
protuberances formed are called tylosoids. This term is also used for outgrowth of neighbouring phloem
parenchyma cells into the sieve tube elements in phloem. Phloem parenchyma cells invade the lumen of the
sieve tubes. Tylosoids differ from tyloses since they do not grow through the pits in the walls.
Reaction Wood
The wood which is formed in response to the inclined or crooked position of a branch is known as reaction
wood. Often many forces of nature like strong winds, soil slippage on hillsides and floods also lead to the
formation of reaction wood. Hence the special wood which is formed as a response to counteract forces of
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various kinds in leaning trunks and branches is reaction wood. In the conifers it develops on the underside of the
branches and is known as compression wood, while in the dicots the reaction wood is formed on the upper side.
Reaction wood develops in the roots too. Experimental work has shown that the stimulus of gravity and the level
of growth hormones like auxins are responsible for the development of reaction wood. Tension wood in dicots is
formed where auxin concentration is low. On the other hand, compression wood formation occurs in regions of
high auxin concentration.
Compression wood is produced by the increased activity of the vascular cambium on the lower side of stem. It is
recognisable by an increased quantity of wood in the compressed area and by the presence of eccentric growth
rings (Fig. 35). Parts of growth rings located on the lower side are generally much wider than those on the upper
side. Typical compression wood is 15% to 40% heavier than normal wood. Also, in comparison with the normal
wood, there is a more gradual transition between the early and late wood. Tracheids present in the compression
wood are shorter in length. In a cross section of the wood, tracheids of compression wood appear to be more
rounded than those of the normal wood and intercellular spaces may be present, which are rare or absent in the
normal wood of conifers and dicots. The walls of the tracheids are slightly thicker as compared with those of
normal wood and they have a higher lignin and cellulose content. The innermost layer-S3 is absent while the
inner face of S2 is deeply grooved. These modifications make the compression wood heavier and more brittle in
comparison with normal wood. Its lengthwise shrinkage upon drying is often at least ten times more than normal
wood. Hence, upon drying, lumber containing compression wood twists and is virtually useless except as fuel.
In the dicots, tension wood, as stated earlier, is produced by the increased activity of the vascular cambium on
the upper side of the stem and as in the compression wood; it is recognisable by the presence of eccentric
growth rings. Histologically, tension wood is characterised by the presence of gelatinous fibres which contain a
special innermost G - layer (gelatinous layer). This inner layer is thick and highly refractive. In addition to the G
- layer, all the other three layers of the wall S1, S2 and S3 may be present or it may have only S1 and S2 layers
or only S1 may occur. The G-layer has little or no lignin or hemicelluloses, hence the cellulose microfibrils are
only loosely interconnected. In the tension wood, in general, there is less lignin and more cellulose. Gelatinous
fibres are normally found on the upper side of the branch in the thickened part of the eccentric growth rings.
Tension wood may have gelatinous fibres occurring in a clustered manner or in a scattered manner where the
fibres are either in small groups or individually present as idioblasts. The former condition is known as compact
tension wood and latter as diffuse tension wood. Another feature of tension wood is the presence of fewer
vessels that have a reduced diameter.
Lumber containing tension wood, twists out of shape on drying.
SECONDARY PHLOEM
The secondary phloem like the secondary xylem consists of both an axial and a radial system. Both these
systems are derived from the fusiform and ray initials respectively of vascular cambium. In comparison with
secondary xylem, secondary phloem constitutes a much less prominent part of the plant body. Vascular
cambium produces much smaller quantities of phloem than the secondary xylem. In several dicot species,
growth rings may also be observed in the phloem, though these are less distinct that those seen in xylem. These
growth rings in the phloem appear due to the differences in the cells produced in the early and late season.
Phloem cells formed in the early season are conspicuously radially extended, whereas those formed at the end of
the season are flattened. These rings become obscured after a few growth seasons due to the obliteration of the
sieve elements. The non-functional phloem is eventually separated from the axis by the formation of periderm.
This is one reason why phloem cannot be used as an indication of the age of the plant, since successive rings of
secondary phloem get included in the periderm (dealt later in the chapter). Tangential bands of fibres are formed
in the secondary phloem of many Gymnospermous and dicot species. However, the number of these is not
constant and therefore these too cannot be used for estimating the age of the tree.
Like the primary phloem, the secondary phloem is also responsible for the long distance transport of organic
nutrients and metabolites. Besides this function, as mentioned in the preceding paragraph, it is one of the main
constituents of the periderm and is therefore responsible for protecting the plant body from microbes, insects
and other animals.
The secondary phloem may be quite rich in secretory tissues, possibly due to the fact that it plays a protective
role in the plant. Extensive duct systems may occur in the phloem. One of the most important ducts in the
phloem are the laticifers. The para rubber tree, Hevea brasiliensis is tapped for its latex to produce rubber. Resin
canals of conifers are a source of resin, which is distilled to make turpentine with the residue forming rosin.
During the dormant periods, secondary phloem also has a major role in the storage of material. This function is
of special importance in the deciduous dicots where new leaves must be produced within a short time in spring.
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The stored substances in the phloem form a source of large amount of carbohydrates and nitrogen required for
this purpose.
Secondary phloem of conifers
The secondary phloem of conifers is rather simple and is less variable than it is in dicots. The axial system
contains sieve cells, albuminous cells and phloem parenchyma cells. Fibres and sclereids may also occur. The
sieve areas are located predominantly on the radial walls, so sieve cells of similar age are united. The sieve cells
and their associated albuminous cells typically constitute a large proportion of the axial phloem while it is still
actively conducting, but after the sieve cells stop transporting, they and the albuminous cells collapse and shrink
to being just minor components. The sieve elements are long cells having numerous sieve areas, commonly
restricted to the radial faces. The parenchyma cells may be single or may form strands. Fibres occur in
Cupressaceae and Taxodiaceae where they form alternating bands with phloem parenchyma and sieve cells. No
fibres are present in Pinaceae, but cells have thick non - lignified walls. Resin ducts are present in some
conifers. For e.g. Abies balsamea. It is the source of resin called Canada balsam which is used as a mounting
medium in making permanent microscopic slides, since its refractive index is the same as that of glass. At a
given time, the conifer phloem appears as a narrow band in a section. The sieve cells in most of the conifer
species secondary phloem function for only one season and then collapse. Hence, approximately one growth
layer is in an active state. However, in some species like Abies balsamea, Picea and Juniperus, the sieve cells
may resume conduction activities in the spring after remaining viable, though dormant during winter season.
The rest is no longer conducting and appears in a collapsed and distorted state, especially when fibres are
absent. The axial phloem parenchyma cells enlarge in the non-conducting phloem and remain alive until being
cut off during periderm formation. The non-conducting phloem continues to function for storage and protection.
Nitrogenous compounds and sugars are stored in this region.
The radial system of secondary phloem contains phloem parenchyma in the form of tall uniseriate rays. They
may contain solely storage parenchyma or they may have albuminous cells, tannin cells, resin canals or other
secretory structures. Ray parenchyma cells, like the axial parenchyma cells, remain alive and active in the nonconducting phloem. If albuminous cells are present here, they collapse in the non-functional phloem.
Secondary Phloem of Dicots
As in the secondary xylem of dicots, the secondary phloem also has a vertical and a horizontal system. The
vertical or the axial system is comprised of sieve tube members, companion cells, phloem parenchyma, fibres
and sometimes sclereids, whereas the horizontal or the ray system is made up of parenchyma rays of different
sizes.
In some species, fibres of the axial system may be scattered either individually (Litsea) or in small cluster
(Liriodendron) throughout the phloem parenchyma. In Vitis and Magnolia, fibres occur in tangential bands
alternating with bands of sieve tubes, companion cells and phloem parenchyma. Fibres in Vitis are septate in
nature. They are living cells, concerned with storage of starch. No fibres are present in Aristolochia. In Carya,
which has a hard bark, the fibres constitute the largest portion of phloem, and they include scattered groups of
other phloem elements.
The secondary phloem may be storied or non-storied depending on the characteristic of the cambium. Storied
phloem is formed in plants having storied cambia (Robinia) and non storied phloem develops in species having
non- storied cambia. The latter develops in a majority of species (Quercus, Betula, and Juglans). Sieve tube
members tend to be rather long with oblique end walls having large compound sieve plates, while the side walls
have much smaller sieve areas. The sieve elements in more or less definitely storied phloem have slightly
inclined or almost transverse end walls and their sieve plates have few sieve areas or only one.
Phloem parenchyma rays in the axial system are similar to the xylem rays of the same plant. Sclereids or
sclerified parenchyma with crystals may be present in these rays. Many of the parenchyma cells containing
crystals sometimes become sub-divided to form chambers, each of which contains a single crystal. Crystals,
sclereids, secretory structures, laticifers and other cells containing special substances may occur in the variously
sized rays of the horizontal systems.
The secondary phloem of dicots is typically active in conduction for one year, after which the sieve tube
members and companion cells collapse (Fig. 36). However, in some species, the phloem may remain functional
for two years (Vitis) or more (Tilia). Included phloem (formed as an anomaly) found in members of
Nyctaginaceae, Chenopodiacceae and other families remains functional for many years. Thick layers of callose
called - definitive callose is deposited on the sieve areas as a sieve tube stops functioning. In many genera Bombax, Vitis, Antiaris, the non-functioning sieve tube members become filled with tylosoids. These are
protrusions from the adjacent parenchyma cells into the sieve tube members. Protoplasts of these cells get
protruded into the sieve tube members. Hence, this is one observation that is made in the non-functional phloem.
15
Most often, sieve tubes are either in a collapsed state and crushed in the non-functional phloem or they remain
open and become filled with gases.
In older parts of the phloem, the rays may become dilated in response to the increase in the girth of the axis.
Enlargement in the diameter of the axis occurs with the increase in the diameter of the secondary xylem formed
by enlarging cambial ring. Due to this, all of the peripheral regions of secondary phloem are pushed outward. In
order to avoid tearing, this tissue must expands. This expansion is called dilatation which is brought about by
the formation of dilatation tissue. This is brought about by two methods. The first is by the formation of
proliferation tissue formed by the divisions and subsequent enlargement of axial parenchyma. The second
method is by the formation of expansion tissue which is formed by renewed divisions in the ray parenchyma
(Fig. 37).
PERIDERM
Periderm formation usually follows the initiation of the formation of secondary xylem and secondary phloem in
most woody stems of gymnosperm and dicots (Fig. 38). Some monocots too have periderm while a few others
form a different kind of secondary protective tissue. In most of the monocots, however, the epidermis persists
throughout the life of the stem as a protective layer. It may become suberised and sclerified and provides
protection. Periderm is a protective tissue that is secondary in origin. It replaces the epidermis in stems as well
as in roots that get increased in circumference due to secondary growth. In many short lived herbaceous plants,
the epidermis may not be replaced by periderm. The age at which development of periderm occurs in the shoot
is variable. Its formation starts within two weeks of growth of the shoot in Acacia, while in Psidium it occurs
within 6-8 weeks of the shoot formation and in Mangifera, periderm development occurs only when the shoots
are 20 weeks old.
Periderm development also occurs along surfaces that are exposed after the abscission of leaves, branches or
even fruits. Periderm formed near injuries or wounds and damage caused mechanically or by parasites is termed
as wound periderm or wound cork. In some taxa periderm formation may also occur in the xylem, and this is
termed as the interxylary cork.
Structurally, the periderm is a composite structure consisting of three parts: phellogen or the cork cambium,
phellem or the cork and phelloderm or the secondary cortex. Due to the insertion of non-living phellem between
the living tissues, all tissues exterior to it die.
Phellogen -It is a secondary meristematic tissue, which like the vascular cambium is a lateral meristem. It is a
secondary meristem because the differentiated cells acquire the ability to divide again. It is relatively simple in
structure in contrast to the vascular cambium as it has only one kind of cell in it. These cells appear rectangular
in outline in a cross section (Fig. 39). In longitudinal sections, the phellogen cells are polygonal or rectangular in
outline. No intercellular spaces are present in the phellogen, except in the regions where lenticels develop.
Protoplasts of phellogen cells may contain chloroplasts, tannins (depending on the site of its origin) and also
variously sized vacuoles.
The site of first phellogen differentiation varies in the taxa. The cork cambium, as mentioned above arises when
living parenchyma cells resume mitotic activity and become meristematic. This conversion can happen in any
layer of cells in the stem. The first phellogen may be formed in the epidermis itself (Quercus, Malus, Nerium,
Pyrus) hypoderms (Populus, Ulmus, Juglans) ; in the cortex (Pinus, Aristolochia, Robinia); or in the phloem
(Vitis, Punica). In the potato tuber, it may develop in the epidermis or in the subepidermal layer, though the
phellogen formed from the epidermis functions for a short period only. In certain species, such as Artemesia,
Achillea fragrantissima and Epilobium augustifolium, phellogen develops in the parenchyma of xylem. In these
plants, layers of cork cells, called interxylary cork are formed in secondary xylem, on the borders of the growth
rings. This feature, especially when accompanied by the cessation of activity in certain portions of the cambium,
results in the splitting of stems. The first phellogen in the roots is usually formed in the pericycle.
The first formed phellogen remains active for sometime. Subsequent phellogens are differentiated in the deeper
tissues. If the first phellogen is from the hypodermis then the next one will be differentiated from the cortical
cells. As more and more secondary vascular tissues are produced by the cambium, the phellogens and the
primary tissues in which they are present are pushed outward and stretched tangentially. Eventually, the existing
phellogens are replaced by new ones, mostly in the secondary phloem. At this point, a balance seems to have
reached in most plants, where new cork cambia initiate at more or less the same rate as new secondary phloem is
produced.
Phellem: The cells of the phellem or the cork appear to be arranged in compact radial rows when seen in crosssections (Fig. 39). No intercellular spaces occur and these cells are flattened radially. The phellem cells
16
produced by the phellogen do not undergo any further mitotic activity and often, grow very little. The primary
wall of the cork cells is made of cellulose, though it may get lignified at times. The main component of the walls
is suberin, which is a fatty substance. The inner wall surfaces are lined by suberin lamellae, consisting of
alternating layers of suberin and wax, which make the tissue highly impermeable to water and gases and it
withstands the action of acids. The walls of cork cells vary in thickness. In plants or organs that require
maximum protection, the phellem cells may have very thick walls and can even become sclerified, e.g.
Leptocereus. Besides becoming sclerified there are also cork cells in different species that become filled with
ergastic substances, tannins, resins and very often-crystals.
Commercial bottle cork is obtained from Quercus suber. Very minute pores are found in the wall of the bottle
cork cells. These pores develop from plasmodesmata and get blocked by a dense material. The cell lumen of
cork cells is filled with air, which is why it is easily compressed. It is elastic in nature and is formed in thick
layers on the trunks of Quercus suber trees. This kind of elastic nature of the cork is lacking in cork of other
genera.
In many species, the phellem is made of the usual suberized cells and also an unusual type of cork cells called
phelloids. Phelloids are unusual because they have non - suberized walls and like the cork cells, their walls may
be thick or thin. These phelloids may later get differentiated into sclereids. In many plants, two common types
of cork cells are formed – one, that are hollow, thin-walled and somewhat widened radially and those which are
thick- walled and radially flattened. Both these types occur in Betula and Abies. In Betula where layers of thickwalled phellem cells alternate with the layers of thin-walled, it can be peeled like sheets of paper.
Phelloderm – Cells of phelloderm are living at maturity and lack suberin lamellae in their walls. They resemble
the cortical parenchyma and may be distinguished from them by their arrangement in radial rows (Fig. 40B).
The amount of phelloderm or secondary cortex produced is in smaller amounts than phellem and it is not
produced at all in some taxa, though substantial quantities of cork are formed. In some cases, phelloderm is an
important tissue that contains chloroplasts and is therefore capable of photosynthesis. These cells may also store
starch. Secretory ducts or other special structures like sclereids may be present among the cells of the
phelloderm in some plants.
Bark, Rhytidome
The term bark is used loosely to refer to the outer protective tissues in the stem as well the root after the
formation of periderm. It is a non-technical term which differs from the periderm. The periderm is made up of
phellem, phellogen and phelloderm where as the bark includes all the tissues exterior to the vascular cambium.
Therefore, bark includes periderm or periderms, secondary phloem, primary phloem as well as cortex. During
each growing season, the vascular cambium adds secondary phloem to the bark (as well as secondary xylem to
the axis). Usually the amount of secondary phloem produced each year is much lesser than secondary xylem.
The functional phloem is the innermost region of the living bark. With the development of new phellogen
occurring interior to the existing periderm, all tissues exterior to it die. Since the new phellogen will form the
innermost periderm the newly formed phellem of this periderm will cut off the supply of water and nutrients to
the outer living cells and as a result the latter will die. Hence, the bark may be divided into an inner bark and an
outer bark (Fig. 41). All the tissues exterior to the innermost phellogen constitute the outermost bark, while the
inner bark includes the living tissues of the innermost periderm and all cells interior to it, though outside the
vascular cambium. The outer bark is also known as the rhytidome. Rhytidome is especially well developed in
old stems and roots of trees. Subsequent periderms that are formed may be in the form of entire cylinders like
the first one. In plants (Vitis, Lonicera) showing the formation of complete successive periderms, approximately
concentric rings develop around the axis (Fig. 42). This may be often referred to as the ring bark. However, in
many plants, the additional phellogens which differentiate are not complete rings, and as a result, the periderm
appears like a scale or a shell. Many such scales- like periderms are formed that cover the tree trunk, as in
Pinus. This is also known as scale bark and is more common in occurrence.
Much variation is observed in the outer appearance of the bark on stems. The morphology of bark, its texture,
constituent cells, arrangement of lenticels in the bark are some of the features that are used as taxonomic
characters.
The bark and the rhytidome besides being protective in nature also helps in removing pathogens. Since, bark is
shed periodically, the layers that fall may carry with them whatever is present in them like bacteria, fungi or
insects.
17
Wound Cork
Whenever there is an injury, living plant tissue is exposed. Since the protective epidermis or periderm is
removed, the surface is exposed to desiccation or it may get infected by various pathogens like bacteria, fungi or
other microbes. In order to prevent such infections, wound periderm formation occurs. The first step in this is
the development of a closing layer. The living cells beneath injured and dead or necrosed cells get deposited by
suberin and lignin and form the closing layer. This layer provides an immediate barrier and prevents loss of
water from the cut surface as well shields against the entry of pathogenic microbes. Formation of the closing
layer is followed by the differentiation of a new phellogen from cells beneath it. This layer initiates the
production of phellem – forming the wound periderm right below the closing layer. After the formation of the
wound periderm, the closing layer gets isolated and dies and may soon fall off. Wound cork formation can
occur on all parts of a plant. These cork cells are similar to the cork cells formed in the normal periderm, but in
certain taxa like Ipomoea batatas, the suberin content is much lower. Similarly, it is seen that monocotyledons
are less responsive to wounding than the dicotyledons. A thin suberized (Zingiberales) or lignified (Palmae,
Poaceae) closing layer may only be formed or healing may occur by the development of both closing layer and
wound periderm (Liliales, Araceae, Pandanaceae). Environmental factors like low temperatures and low
humidity can also affect and delay the formation of wound periderm.
Protective tissue in monocots
In a majority of monocotyledons (Cyperaceae, Juncales, Marantaceae, Gramineae, Typhaceae and other taxa)
where no secondary growth occurs the epidermis remains the only protective tissue throughout the life of the
plant. To be effective for such long periods, the epidermal cells may undergo sclerification. Often, the surface
layers of the stem may become suberized.
In woody monocots, two types of periderm have been observed. The first type of periderm formed is similar to
the type formed in the dicotyledons and Gymnosperms. This periderm is reported in the members of the families
Strelitziaceae, Heliconiaceae, Musaceae and Lowiaceae. The second type of protective tissue observed is known
as storied cork. Pockets of parenchyma cells in the outer ground tissue become meristematic in nature and
divide periclinally. Hence, the initials here do not form a regular uninterrupted cylinder. The daughter cells
formed undergo little expansion and their shape resembles the parent dividing cells. Since these cells are
arranged in regular tiers the periderm is known as storied cork (Fig. 43). Storied cork is found to occur in the
genera belonging to the families Zingiberaceae, Bromeliaceae and Commelinaceae.
Lenticels
Lenticels are portions of the periderm where numerous intercellular spaces are present in an otherwise
compactly arranged phellem layer. The part of the phellogen giving rise to the lenticel (lenticel phellogen), also
has intercellular spaces. Due to the presence of a large number of intercellular spaces in the lenticels, it is
believed that they permit entry of air and allow gaseous exchange, similar to that of the stomata (Figs. 40; 44).
Lenticels are present in the periderm of both roots and stems. They are found on the surfaces of fruits like
apples, (where the peel is the bark), and also pears in the form of small dots.
Mature lenticels externally appear as more or less lens - shaped structures and they are convex on both the
interior as well as the exterior side. On the surface, it appears as a protruding mass of loose cells. The lenticels
may be either vertical or horizontal in position. Much variation exists in their size as well, as in some genera
they may be microscopic in nature while in others, they may be as much as one centimeter or more in length.
Their arrangement is also variable; they may be scattered or arranged in vertical or horizontal rows or present in
some other way. Lenticels are usually positioned opposite to the wide phloem rays. This permits a relatively
easy exchange of gases. If they were located between phloem rays, then the axial fibres of the phloem would a
barrier to this gaseous exchange. Lenticel development may occur under a stoma or a group of stomata in certain
species. However, where the number of stomata is less, lenticels may be formed between them.
Initiation of lenticel formation begins with the first periderm or even before. When the two develop
simultaneously, then the phellogen contains two almost indistinguishable regions, one producing compactly
arranged cells of phellem and the other producing complementary cells which are also known as filling cells.
The main difference between the two is that the tissue of the lenticel has a lot of intercellular spaces and their
walls may be thinner. On the inside, the phellogen forms the phelloderm. Since the volume of the
complementary cells is larger, they push the overlying epidermis and any cortical cells (if present) outward and
these layers eventually rupture. When the first lenticel arises before the first periderm, then the initiation usually
occurs due to division in the parenchyma cells below a stoma or a group of stomata. These dividing cells form
the phellogen of the lenticel.
18
Complementary cells produced are of two types - suberized or non- suberized. Also, these cells are constantly
removed from the surface of the lenticel due to weathering. Mainly two types of lenticels are found in the plants.
In the first type, the complementary tissue that is produced by the phellogen is composed of relatively compact
cells having quite strong connections between them, while in the second , the complementary cells have very
loose connection and as a result are powdery in nature.
In many genera, these complementary cells are suberized, as in genera like Liriodendron, Persea, Malus,
Magnolia and Salix. However, in certain other genera like Tilia, Quercus and Sambucus, the complementary
cells are more or less loosely arranged and are non- suberized in nature, though at the end of the season a layer
of compact cells is formed. In the second type of lenticel, the complementary cells and non-suberized and are so
loosely interconnected, that they have an almost powdery consistency. Here, almost no attachment exists
between the complementary cells, and due to the loose arrangement, these cells are easily removed from the
lenticel. These cells are held together by alternating layers of compact suberized tissue which forms the closing
layers (Fig. 45). These closing layers hold the loose complementary cells together and are continuously ruptured
by the pressure exerted from below by the constant formation of more and more filling cells by the lenticel
phellogen. This type of lenticel is regarded as a specialized one and it occurs in Prunus and Betula besides other
genera.
.
19
ILLUSTRATIONS
Fig. 1 A vegetative twig of Croton
Fig. 2 Diagrammatic representation of shoot apex showing phytomeres
20
Fig.3 L.S. of shoot apex of Marsilea with a single apical cell
Fig.4 Diagrammatic representation of L.S. shoot apex of Pinus
Fig. 5 L.S. Shoot apex of Coleus showing tunica and corpus
21
Fig.6 Diagrammatic representation of L.S. shoot apex showing the
various surface layers of shoot meristem,
Fig.7 Line diagrams depicting the shoot apex in minimal, maximal
and again minimal phase of development.
22
Fig. 8 T.S. of Medicago stem (a sector)
Fig.9 T.S. of outer cortex showing a collenchymatous hypodermis.
23
Fig. 10 T.S. stem of Helianthus showing vascular bundles in a ring.
Fig. 11 Outline diagrams of different types of vascular bundles found in
vascular plants. A – conjoint , collateral, open and endarch V B.
B – conjoint, collateral , closed and endarch V B. C- conjoint,
bicollateral, open and endarch V B . D- amphicribral V B.
E- amphivasal V B
24
Fig.12 Tracheary elements showing different types of thickenings
in their longitudinal walls
Fig. 13 T.S. vascular bundle of Zea mays showing
protoxylem lacuna.
25
Fig.14 Different types of steles found in pteridophytes.
A
B
C
D
Fig. 15 Diagrammatic representation of different steles. A- Protostele. B- Siphonostele with no leafgaps
C - Siphonostele with leafgaps. D- Eustele.
26
Fig.16 Diagrammatic representation of unilacunar node (1,2,4),
trilacunar node (3). Development of trilacunar and multilacunar nodes
from unilacunar- 2-trace node.
Fig.17 Multilacunar node
27
Fig. 18 Evolution of Unilacunar and multilacunar node from trilacunar node.
Fig. 19 Evolution of different nodes from a trilacunar node.
28
A
B
C
D
E
Fig. 20 Diagrammatic representation of stem development in a woody dicot.
29
Fig. 21 T.S. dicot stem (sector) showing some secondary growth
30
Fig. 22 T.L.S. storied cambium (1) and non-storied cambium (2)
Fig. 23 Vascular cambium in relation to derivative tissues.
31
Fig. 24 Multiplicative divisions in fusiform initials.
A - anticlinal division. B - pseudotransverse division
Fig. 25 Divisions in fusiform initials (A—E)
and intrusive growth in fusiform initials (F, G)
32
Fig. 26 Block diagram of secondary xylem of Pinus
Fig. 27 T.S. Pinus wood showing tracheids.
33
A
B
Fig. 28 Conifer wood in R.L.S. (A), and T.S. (B) showing resin ducts
Fig. 29 Diagrammatic representation of pit-pairs.
A
B
Fig. 30 T.S. ring-porous (A) and diffuse- porous (B) woods.
34
Fig. 31 T.S. wood showing growth rings. Sapwood and heart wood are also distinct.
Fig. 32 Diagrammatic representation of distribution of axial parenchyma (dotted) in wood.
35
Fig. 33 R.L.S. heterocellular ray showing both procumbent and upright cells.
A
B
Fig. 34 Inflated balloon like protuberances --- tyloses--- in vessel members as seen in transverse (A)
and longitudinal (B) sections.
36
Fig. 35 T.S. stem showing reaction wood and eccentric growth rings.
Fig. 36 L.S. Secondary phloem showing collapsed sieve tube members and companion cells.
37
A
B
Fig. 37 Dilatation tissue formed in secondary phloem (A). This is also observed in the three year
old stem of Tilia (B)
Fig. 38 Outline diagram of a sector of T.S. stem showing initiation of periderm formation.
38
Fig. 39 T.S. periderm showing phellem, Phellogen and Phelloderm.
Fig. 40 Stages in development of periderm and lenticel.
39
Fig. 41 T.S. of periderms showing outer and inner bark
Fig. 42 T.S. of periderm and Rhytidome.
40
Fig. 43 T.S storied cork.
Fig. 44 T.S. lenticel
41
A
B
Fig. 45 L.S. of lenticels showing alternating layers of complementary cells and closing layers (A,B)
42
ILLUSTRATIONS CREDIT
1. Eames, A. J . and MacDaniels, L.H.1947. An Introduction to Plant Anatomy. 2nd Edition.
Figure 29 adapted from Figure 22.
2. Esau, K.1977. Anatomy of Seed Plants. John Wiley and sons.
Figure 4 adapted from figure 16.17; figure 5 adapted from figure 16.13; figure 23 adapted from figure
10.1;
Figure 25 adapted from 10.5; figure 28 adapted from 9.3; figure 32 adapted from 9.9; figure 37A
adapted
from figure11.19A; figure 39 adapted from 12.1A; figure 42 adapted from figure 12.7; figure 43
adapted
from figure 12.9.
3. Fahn, A.1985. Plant Anatomy. 3rd edition. Pergamon Press.
Figure 3 adapted from figure 31.2; figure 13 adapted from figure 108.1; figure 16 adapted from figure
102.5; figure 22 adapted from figure 164; figure 45B adapted from figure 209.2.
4. Mauseth, J.D. 1988. Plant Anatomy. Benjamin Cummins Press.
Figure 24 adapted from figure 14.8; figure 33 adapted from figure 15.15; figure 45A adapted from
figure
17.19
5. Pandey, S.N., Misra, S.P. and Trivedi, P.S. 2006. A Textbook of Botany Volume 2. Vikas Publishing
House Pvt. Ltd.
Figure 14 adapted from figure 36.11.
6. Raven, P.H. , Evert, K.F. and Eichhorn, S.E. 1999.Biology of Plants. 5th Edition. W. H. Freeman & Co.
Worth Publishers, New York.
Figure 1 adapted from figure 26.1; figure 2 adapted from figure 26.3; figure 8 adapted from 26.10b;
figure
12 adapted from figure 19. 4b; figure 15 adapted from figure 19.5; figure 20 adapted from figure 27.6;
figure 21 adapted from figure 27.7; figure 26 adapted from figure 27.20; figure30 adapted from figure
27.24; figure 34 adapted from figure 27.29; figure 35 adapted from figure 27.30; figure 36 adapted from
figure 27.16; figure 37B adapted from figure 27.9c; figure 38 adapted from figure 27.8; figure 40B
adapted
from figure 27.10; figure 41 adapted from figure 27.13; figure 44 adapted from figure 27.11.
7. Singh, V., Pande, P.C. and Jain, D.K. Anatomy of Seed Plants. Rastogi Publications.
Figure 11 adapted from figure 13.11; figure 17 adapted from figure 13.27D; figure 18 adapted from
figure
13.28; figure 19 adapted from figure 13.30.
8. Uno,G., Richard, Storey and Randy, Moore. 2001. Principles of Botany.
Figure 9 adapted from figure 4.23; figure 10 adapted from figure 8.1; figure 27 adapted from figure
8.16;
figure 31 adapted from figure 18.17a.
43
SUGGESTED READINGS
1.
Cutter, E.G. 1978. Plant Anatomy: Experiment and Interpretation. Volume I and II. Edward
Arnold (Publications) Ltd., London.
2.
Eames, A.J. and MacDaniels, L.H.1947. An Introduction to plant Anatomy. 2nd edition. McGraw Hill, New York.
3.
Esau, Katherine. 1977. Anatomy of Seed Plants. John Wiley and Sons.
4.
Fahn, A.1985. Plant anatomy. 3rd. edition. Pergamon Press.
5.
Mauseth, J.D. 1988. Plant Anatomy. Benjamin -Cummins.
6.
Raven,P.H., Evert, K. F. and Eichhorn, S. E. 2005. Biology of Plants. 7th. Edition. W.H. Freeman
and Co. Worth Publishers, New York.
7.
Uno, G., Richard, Storey and Randy Moore. 2001. Principles of Botany.
tice Hall. New Jersey.
44