Anomalous Secondary Growth
Lesson Prepared Under NME ICT
National Coordinator: Prof.S.C. Bhatla
Discipline: Botany
Paper: Plant Anatomy
Lesson: Anomalous Secondary Growth
Lesson Developer: Dr. Pooja Gupta
Department/College: Ramjas College
Lesson Reviewer: Dr Basudha Sharma
Lesson Editor: Dr Rama Sisodia, Fellow in Botany ILLL
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Anomalous Secondary Growth
Table of Contents
Chapter: Anomalous secondary growth
•
Introduction
•
Anomalous secondary growth in dicot stem
•
Unusual position of the vascular cambium
• Cambium in the form of folds or ridges
• Cambium in the form of separate strips
•
Unusual activity of the vascular cambium
• Formation of unusually large amount of secondary
vascular tissues only in the region of vascular
bundles
• Formation of wide medullary rays
• Formation of ridges and grooves
• Formation of phloem wedges
• Formation of irregular ‘islands’ or patches of
parenchyma in xylem
•
Accessory cambium formation and its activity
•
Formation of included or interxylary phloem
• Formation of included phloem by formation of arc
cambium
• Formation of included phloem by sudden change in
activity of cambium
•
Development of interxylary cork
•
Anomalous secondary growth in monocot stem
•
Anomalous secondary growth in dicot root
• Storage roots
• Aerial root
•
Summary
•
Exercise
•
Glossary
•
Further Reading
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Anomalous Secondary Growth
Learning Objectives
To know:
What is anomalous secondary growth?
What is its significance?
Why it occurs?
In which plant groups it is found?
Introduction
Anomalous secondary growth refers to the deviation of the secondary growth from
the normal type of growth. It is also known as, abnormal or more appropriately
unusual secondary growth, as the term encompasses some less common type of
secondary
growth
patterns.
According
to
International
Association
of
Wood
Anatomists (IAWA), the term ‘anomalous’ is no longer in use, instead term ‘unusual’
should be employed. However, still these terms are continually been used
synonymously. Though secondary growth is an exclusive feature of dicotyledonous
plants, but there are some monocotyledons that also show secondary growth.
Significance of Anomalous Secondary Growth
It is believed that plants show anomalous secondary growth primarily because of two
reasons:
i)
As an adaptation to the environment – Some anomalies in the plant
structure arise in response to the environment to cope with it. Such forms
are termed as adaptive type. This includes the plants with:
a) climbing habits – This is found in climbers and lianas. The climbers
should have soft tissue like parenchyma or secondary phloem in
abundance to promote their flexibility and twining or twisting habit.
These tissues may also split the solid woody cylinder into strands
helping the plant to climb, e.g. Aristolochia and Tinospora show fluted
vascular bundles; Bignonia show phloem wedges; Leptadenia and
Thunbergia show presence of interxylary phloem.
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A flattened stem is sometimes encountered in climbers which
helps the plant to hold onto the support while climbing, e.g. as seen in
Bauhinia.
In
some
other
climbers,
such
as
Serjania,
Thnouia,i
Ichthyoctona and Paullinia, cambium develops in the form of separate
strips and the mature stem has many distinct vascular bundles which
develop their own periderm and may progressively get separated from
each other. The stem thus, seems to be made up of a number of
strands of smaller stems closely appressed to each other, resembling
strands in a rope. This provides strength to the stem against extension
and breakage facilitating twisting and twining.
b) storage roots - Many plants have storage roots where the reserve food
material is stored in the parenchymatous tissue. A considerable
amount of storage parenchymatous tissue is formed as a result of
anomalous secondary growth in them which is considered to be an
adaptation to their storage function, e.g. Beta vulgaris, Raphanus
sativus, Ipomoea batatas and Daucus carota.
c) floating habits – The parenchymatous tissue when encloses a lot of air
space (referred as aerenchyma) can provide buoyancy to the aquatic
plant, e.g. in Jussiaea, cork cambium produced at the time of
secondary growth gives rise to parenchyma only that help in
buoyancy.
ii)
Variation in the cambial activity – In nature there is variation in the
position, development, behaviour and/or nature of cambium found in
some plants leading to varied structural organizations. Such forms, with
structural anomalies which are not because of the environment, are
referred to as non-adaptive type. This is found in many plants such as
Boerhaavia,
Mirabilis,
Amaranthus,
Chenopodium,
Dracaena, etc.
Anomalous Secondary Growth in Dicot Stem
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Bougainvillea,
Anomalous Secondary Growth
Anomalous or unusual secondary growth may occur due to:
1. Unusual position of the vascular cambium
2. Unusual activity of the vascular cambium
3. No development of usual cambium or if so happens, its replacement by other
accessory cambium formation and its activity
4. Formation of included or interxylary phloem
5. Development of interxylary cork
For better understanding, the anomalous secondary growth may be studied under
the above stated categories in some representative plant species:
Unusual position of the vascular cambium
The vascular cambium normally lies in between primary xylem and primary phloem.
But sometimes, in plants such as Thinouia sp., Serjania sp., Paullinia sp., Bauhinia
langsdorffiana, etc. cambium may be present elsewhere and its location may not be
well differentiated. Cambium in such unusual position shows unusual activity
resulting in anomalous secondary structure.
Cambium in the form of folds or ridges
In the young stem of climbers e.g. Thinouia scandens,, the cambium is thrown into
folds or ridges. At the time of secondary growth, the cambium separates at the folds
and gives rise to separate groups of vascular tissues, resulting in a lobed stem.
Cambium in the form of separate strips
a) This is commonly found in climbers, Serjania Ichthyoctona and Paullinia of family
Sapindaceae. The cambium here originally appears in many separate strips, each of
which surrounds small parts or may be some strands of primary xylem and phloem.
As the secondary growth starts, each cambial strip forms a separate entire ring of its
own and behaves normally by cutting secondary xylem inside and secondary phloem
outside. The mature stem thus has many distinct vascular bundles. Such a stem
seems to be made up of many fused stems. In older stems, the discrete vascular
bundles develop their own periderm and may progressively get separated from each
other. The stem thus, seems to be made up of a number of strands of smaller stems
closely appressed to each other, resembling strands in a rope.
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Figure: T.S. Serjania stem (Per: Periderm)
Distinguishing feature: Cambium is in the form of separate strips; stem has many
distinct vascular bundles; seems to be made up of many fused stems
http://virtualplant.ru.ac.za/Main/ANATOMY/serjania-stem1.jpg
b) Bauhunia langsdorffiana of family Caesalpiniodeae also shows a similar kind of
stem structure. Here, the original cambium ring is broken into strips or sometimes,
even the vascular cylinder formed by this cambium is broken into parts. This
happens due to the excessive production and proliferation of the secondary
parenchyma
by
the
cambium.
The
cambium
produces
xylem
and
phloem
parenchyma in excessive amounts to an extent that vascular bundles get broken up
into several parts resulting in small groups or fissures of xylem including the
cambium itself which formed them.
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Figure:
T.S.
stem
diagrammatic:
A.
Thinouia
scandens;
B.
Serjania
ichthyoctona; C. Bauhinia langsdorffiana
Source: Author
Unusual activity of the vascular cambium
The cambium is normal in position but it shows an abnormal activity leading to
irregular arrangement of secondary tissues. The unusual activity of cambium can be
studied under following sub-heads:
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Formation of unusually large amount of secondary vascular tissues only in
the region of vascular bundles/formation of increased size of vascular
bundles
This may happen when only intrafascicular cambium is active and it forms secondary
vascular tissues in the region of vascular bundles only, e.g. Cucurbita sp. stem.
ADDITIONAL INFORMATION:
ANAOMALOUS SECONDARY GROWTH IN Piper spp.
Piper, a member of family Piperaceae shows an unusual arrangement of vascular
bundles, e.g. in Piper betle, there are two groups of vascular bundles – medullary
bundles and cortical bundles. The medullary vascular bundles are irregularly
arranged and are comparatively bigger than the cortical bundles which lie to the
exterior of an undulating cylinder of sclerenchyma. Another species, Piper excelsum,
also has two cylinders of vascular bundles. Two primary medullary vascular bundles
are at the center of the stem. These are surrounded by an irregular inner cylinder of
similar bundles which is further enclosed by an outer cylinder of vascular bundles in
which cambium develops. As a result of cambial activity, large, radially extended
regions of secondary vascular tissues separated by medullary rays are formed.
Figure: Variation in stem structure in two species of Piper (Piperaceae). (a)
Transverse section of a stem of Piper betle. The stem has an inner, irregular cylinder
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Anomalous Secondary Growth
of primary vascular bundles, and an outer cylinder of smaller bundles to the exterior
of an undulating wall of sclerenchyma. Note also the large mucilage ducts in the pith
and inner cortex. Magnification × 8. (b) The stem of Piper sp. characterized by a
central, irregularly arranged group of primary vascular bundles enclosed by a thick
cylinder of secondary tissues consisting of regions of tracheary tissues capped by
phloem. The secondary tracheary tissues are separated by ray-like regions of
secondary parenchyma. Magnification × 3.3. (c) Detail of the vascular cambium and
its derivative tissues from the stem shown in (b). Magnification × 180.
Source: Beck, C.B. (2010). An Introduction to Plant Structure and Development:
Plant Anatomy for the Twenty-First Century. Second Edition. Cambridge University
Press, New York, USA.
Cucurbita stem (Family Cucurbitaceae) – The young stem is characterized by
presence of two rings of vascular bundles with five bundles in each ring. Each
vascular bundle is open, conjoint and bicollateral with an outer and inner cambium.
At the beginning of secondary growth, only the outer cambia (of both the outer and
inner rings of bundles) becomes active. The parenchymatous cells of the ground
tissue dedifferentiate to become meristematic which in combination with the outer
cambium forms an undulating and poorly marked ring of cambium. It forms the
secondary vascular tissues in the vascular bundle region only. It results in an
increased size of the bundles.
Figure: T.S. Cucurbita stem
http://ebot.library.usyd.edu.au/media/images/thumbs/283.jpg
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Figure: A part of Cucurbita stem showing bicollateral vascular bundle
http://ebot.library.usyd.edu.au/media/images/thumbs/455.jpg
Formation of wide medullary rays
In this case, a normal cambium ring is formed by the union of intra- and
interfascicular cambium, but it shows unusual activity where interfascicular cambium
forms parenchyma only resulting in the formation of wide medullary rays. This is
seen in following plants:
a) Vitis and Clematis stem – Here only interfascicular cambium is active, it cuts off
parenchyma only. Prominent wide rays are formed.
lFigure:T.S. Vitis stem
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http://www.sbs.utexas.edu/mauseth/weblab/webchap16secphloem/web16.2-6a.jpg
b) Aristolochia stem (Family Aristolochiaceae) – This genus is typical for the study of
formation of fluted/fissured/bifurcated xylem. The two constituent cambia of the
normally positioned cambial ring behave differentially. The intrafascicular cambium
cuts off secondary vascular tissue whereas the interfascicular cambia forms
parenchyma cells only. Thus, it is the interfascicular cambium that becomes
anomalous and leads to fluting of vascular bundles.
In young stem, there is a uniseriate epidermis followed by a multilayered
collenchymatous hypodermis. Cortex is made up of collenchyma and parenchyma.
The single layered endodermis is followed by multiseriate sclenchymatous pericycle.
The vascular bundles are open, collateral and arranged in a ring.
At the time of secondary growth, a normal cambium ring is formed by joining of
intra- and interfascicular cambium, but it behaves abnormally. It forms secondary
vascular tissues in the vascular bundle region and parenchymatous tissue only in the
region between the vascular bundles. Hence, the intra- and inter-fascicular cambia
show differential activities. The inter fascicular cambium constantly cut only
parenchyma cells both on outer and inner sides, thus forms ray like parenchyma that
increases the diameter. Thus the medullary rays become broad so that the vascular
bundles are widely spaced. In the older stems, as the vascular tissue expands, the
sclerenchymatous pericycle encircling the bundles gets ruptured and the adjacent
parenchyma cells invade the gaps by intrusive growth. Consequently, it results in the
fissuring of the xylem and bifurcation of the vascular bundles giving them a fluted
appearance. Fluted vascular bundles are the most distinguishing characteristic
feature of old Aristolochia stem. Vessels are generally wide, In some species, e.g. A.
triangularis, the pith increases and the vascular bundles appear fan-like.
Aristolochia species are mostly woody climbers or lianas. These often show
unusual structural features which in some way or the other are adaptations to the
climbing habit of the plant. For example, wide parenchyma rays present between the
vascular bundles provide resistance to the stem against the high pressure winds.
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http://delta-intkey.com/angio/images/aristem.gif
Figure: T.S. Aristolochia stem
Distinguishing feature: Prescence of wide medullary rays
http://nickrentlab.siu.edu/PLB400/images/Figure17_13.jpg
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A
B
T.S. Aristolochia stem to show presence of wide medullary rays
A. T.S. stem – a complete section
http://www.phytoimages.siu.edu/users/Cusman1/2_25_12/Aristolochi
a
B. T.S. stem – a part of Fig. A enlarged
http://www.phytoimages.siu.edu/users/Cusman1/2_25_12/Aristolochi
a3.jpg
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Figure: T.S. Aristolochia stem: A. Diagrammatic; B. A part cellular
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Source: Author
T.S. Aristolochia young stem
1 epidermis, 4 cortex (= 2 + 3 + 5 + 6), 7 + 8 vascular bundles and 8 pith tissue.
Differentiated tissue is already present before secondary growth has been initiated.
http://www.vcbio.science.ru.nl/images/stemgrowth/SGPL0102_StemAristolochiaYou
ngOverview_small.jpg
T.S. Aristolochia old stem
1 cork; 2 cortex; 3 sclerenchyma; 4 phloem; 5 xylem; 6 ray; 7 pith.
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http://www.vcbio.science.ru.nl/images/stemgrowth/SGPL0101_StemAristolochiaOldC
rossOverview_small.jpg
Figure: A part of T.S. Aristolochia old stem
1 cork; 2 cork cambium; 3 collenchyma; 4 parenchyma; 5 sclerenchyma; 6 phloem;
7 Vascular cambium; 8 xylem; 9 pith; 10 ray (parenchyma cells)
Source:http://www.vcbio.science.ru.nl/public/Final-Images/PL_Final512m_101150/PL0122_512mStemAristolochiaOldCrossPortionOverview.jpg
Formation of ridges and grooves
In this case, a normal cambium ring is formed by the union of intra- and
interfascicular cambium, but it shows unusual activity as in certain regions it is more
active while in other regions it is less active.
Bauhinia stem (Family Caesalpiniodeae) – Here, the cambium is active at definite
places, though it differs in different species. In some species, the cambium is active
only on two opposite sides with little or no activity at other remaining sides. This
results in the formation of a flat, ribbon-shaped stem. In B. rubiginosa, a liana, the
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cambium shows abnormally localized activity at four places only resulting in the
ridged and furrowed stem.
Figure: Diagram showing unequal activity of cambium in Bauhinia
http://image.slidesharecdn.com/anomaloussecondarygrowth-091002213204phpapp02/95/slide-12-728.jpg?cb=1254537158
Formation of phloem wedges
This is seen in some lianas, such as Bignonia, Pyrostegia, Doxantha, Amphilobium,
Cyclostoma, Parabigonia, etc.
Bignonia stem (Bignoniaceae) – The young stem has a thick cuticle present over the
uniseriate
epidermis.
Hypodermis
is
mostly
collenchymatous,
sometimes
sclerenchymatous in the ridges and is meagerly chlorenchymatous in the groove. The
parenchymatous cortex is limited by an endodermis on the inner side. The pericycle
is made up of different types of tissues with sclerenchyma below the ridges. Vascular
bundles are arranged in a ring with each being open, conjoint and collateral. There is
distinct pith in the center. At the time of secondary growth, inter- and intrafascicular
cambium join to form a normal cambium ring. Initially, it behaves normally by
cutting more of secondary xylem on the inner side and less of secondary phloem on
the outer side. But soon, at some definite points which are the four diagonal regions,
it behaves abnormally. It becomes unidirectional in nature meaning thereby, that it
cuts off more of secondary phloem on the outer side and little or almost nil amount
of secondary xylem on the inner side. Consequently, four deep phloem wedges of
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irregular width are formed which project themselves into the xylem. A strip of
cambium is seen at the base of each wedge.
After sometime, the central region of the cambium segments which were
behaving normally, start behaving abnormally similar to the unidirectional cambium.
In the same manner, it forms more phloem externally than xylem internally,
resulting in the formation of four more smaller phloem wedges. The mature stem
thus acquires a complex structure of four big and four or sometimes more small
phloem wedges.
The cork cambium arises in the cortex by the process of
dedifferentiation. The sclerenchymatous pericycle is sloughed off after the secondary
growth and the stem may acquire a circular outline.
Figure: T.S. Bignonia capreolata stem
http://www.phytoimages.siu.edu/users/paraman1/3_28_13_3/Upload28Mar13e/Bign
oniaCapreolata3.jpg
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Figure: T.S. Bignonia stem: A. Diagrammatic; B. A part cellular
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Source: Author
Figure:
http://image.slidesharecdn.com/anomaloussecondarygrowth-091002213204phpapp02/95/slide-7-728.jpg?cb=1254537158
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Figure: A part of T.S. Bignonia stem
http://image.slidesharecdn.com/anomaloussecondarygrowth-091002213204phpapp02/95/slide-8-728.jpg?cb=1254537158
Formation of irregular ‘islands’ or patches of parenchyma in xylem
Urtica dioica stem (Family Urticaceae) – The young stem has vascular bundles which
are open, conjoint and collateral arranged in a ring. Early in the growth, a normal
cambium ring is formed that starts behaving normally. But soon after sometime, it
starts acting abnormally at certain places only. It cuts off only parenchyma cells on
the inner side instead of secondary xylem. However, after sometime it resumes back
its normal activity to form secondary xylem internally which comes to lie on the
parenchymatous cells. This process occurs repeatedly a number of times forming
‘islands’ of parenchyma embedded in the secondary xylem. This gives a false
appearance of included phloem.
Accessory cambium formation and its activity
In many genera, a new cambium ring or accessory cambial rings originate(s) in the
cortex or pericycle where either the normal cambium ring is altogether absent, e.g.
Amaranthus, or on cessation of its activiy, e.g. Boerhaavia. The unusually positioned
cambium (referred to as extrastelar in origin and accessory cambium) behaves
unusually resulting in the formation of successive rings of vascular bundles
embedded in parenchyma or conjunctive tissue. This has been seen in Amaranthus,
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Mirabilis and Bougainvillea and Boerhaavia, etc. Here, the first ring of cambium
arises in the pericycle region and it shows unusual activity by cutting off secondary
xylem in patches alternating with parenchyma cells (or conjunctive tissue) on the
inner
side
whereas
externally,
initially
forming
parenchymatous
layers
and
afterwards forming secondary phloem. It forms a complete ring of vascular bundles
and then it stops functioning. A new ring of cambium called accessory cambium is
formed from the parenchyma cut off externally by the earlier cambium. This newly
formed cambium also behaves unusually in a similar manner forming another ring of
vascular bundles embedded in parenchyma and then becomes inactive. Likewise,
more accessory cambia are formed giving rise to successive rings of vascular
bundles.
a) Amaranthus stem (Family Amaranthaceae) – The primary structure of the stem
shows a number of shallow ridges and furrows with a thickly cuticularised single
layer
of
epidermis.
Below
the
epidermis
there
are
alternating
groups
of
collenchymatous and chlorenchymatous cells. The parenchymatous cortex is followed
by endodermis and pericycle in small uneven sclerenchymatous patches. The normal
ring of vascular bundles is absent. Instead there are two rings of medullary bundles
formed
by the
activity of
accessory
cambia. The first
accessory
cambium
differentiates in the pericycle. It behaves unusually by first forming small amount of
parenchyma on the outside, and then cutting xylem alternating with parenchyma on
the inner side and consequently forming phloem alternating with parenchyma on the
outside. As a result, a ring of conjoint, collateral vascular bundles is formed which
gets embedded in parenchymatous tissue. After sometime, this cambium ceases to
function and becomes passive. A second accessory cambium arises from the
parenchyma cut off by the previous one on the outside. It also behaves in a similar
fashion producing a second ring of vascular bundles again included in the
parenchyma, but alternating to the first one. Similarly, numerous accessory cambia
develop consecutively producing consecutive rings of vascular bundles, giving a
scattered appearance in the ground tissue of the stem. The final accessory cambium
ring forms sclerenchyma alternating with xylem internally thus the last ring of
vascular bundles seems to be embedded in sclerenchyma. On maturity, sometimes
the medullary bundles along with some adjoining parenchyma may degenerate
creating cavity.
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Though the normal vascular cambium is not formed but the cork cambium is
formed and function normally.
Figure: TS Amaranthus stem (very young)
http://ebot.library.usyd.edu.au/media/images/view/1739.jpg
Figure: TS Amaranthus stem (old)
http://ebot.library.usyd.edu.au/media/images/thumbs/1749.jpg
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Figure: TS Amaranthus stem (very old)
http://ebot.library.usyd.edu.au/media/images/view/1697.jpg
b) Mirabilis stem (Family Nyctaginaceae) – The young stem is characterized by the
presence of two grooves, one on either side, bearing numerous shoot hairs. It has a
uniseriate epidermis followed by collenchymatous hypodermis and parenchymatous
cortex. A well-defined endodermis and pericycle with sclenchymatous patches can be
distinguished. A number of vascular bundles are seen scattered inner to the
pericycle.
Earlier it was believed that the first accessory cambium develops from the
pericycle in Mirabilis stem, however later it was established that the first cambium
ring develops in the region of outermost vascular bundles. Several cambia arise
successively centrifugally. Each such accessory cambium produces xylem and
conjunctive tissue to the inside, and phloem and conjunctive tissue to the outside.
The resulting tissue gives the appearance of concentric rings of vascular bundles
embedded in conjunctive tissue.
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Figure: T.S. Mirabilis stem
http://ebot.library.usyd.edu.au/media/images/view/1716.jpg
c) Bougainvillea stem (Family Nyctaginaceae) – The stem is circular in outline when
young. It has a uniseriate epidermis covered by a thick cuticle, collenchymatous
hypodermis and a well developed parenchymatous cortex. The ill-defined endodermis
is followed by a pericycle made of parenchyma with intermittent sclerenchymatous
patches.
The primary vascular bundles are seemingly scattered in the ground tissue and
are not arranged in a ring. The first ring of cambium arises from the pericycle thus is
extrastelar in origin. This is followed by formation of successive rings of cambia,
though it was also believed that all the secondary tissue derivatives arise from a
single cambium. Each cambial ring cuts off xylem alternating with parenchyma
internally and, phloem and alternating patches of parenchyma externally. The
parenchyma so formed usually gets lignified, which is then referred to as conjunctive
tissue. Thus, concentric rings of vascular bundles are formed embedded in
conjunctive tissue. In some species, the conjunctive tissue is sclerenchymatous only
and is hardly distinguishable from the tracheary elements of the embedded vascular
bundles. The phloem appears as an isolated patch actually surrounded by the
conjunctive tissue, which is often mistaken to be included phloem.
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T.S. Bougainvillea stem; C-Collenchyma; CZ-Cambial zone; E-Epidermis; SSAccessory cambium; 2P-Secondary phloem; 2X-Secondary xylem
http://image.slidesharecdn.com/anomaloussecondarygrowth-091002213204phpapp02/95/slide-5-728.jpg?cb=1254537158
d) Boerhaavia stem (Family Nyctaginaceae) – Boerhaavia shows anomalous
secondary growth due to anomaly in its primary structure as well as accessory
cambium formation and its activity at the time of secondary growth. The young stem
is typically dicotyledonous in structure with a few unusual features. It is
characterized by a single layered epidermis with numerous hairs, collenchymatous
hypodermis, parenchymatous cortex, an indistinct endodermis and 1-2 layered
parenchymatous pericycle. There are three rings of vascular bundles which are
primary in origin – the innermost two large medullary bundles, middle ring composed
of 6-14 loosely arranged bundles and the outermost ring of 15-20 small bundles. The
central medullary bundles and the middle ring bundles show restricted secondary
growth and only small increase in size, though the intrafascicular cambium in these
bundles behave normally. It forms secondary xylem towards inside and secondary
phloem towards outside with primary phloem pushed to lie only as a cap like
structure towards periphery.
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The cambium of the outermost ring of the vascular bundles forms a complete ring
at the time of secondary growth by the union of the inter- and intrafascicular
cambium.
However,
the
two
constituent
cambia
behave
differentially.
The
intrafascicular cambium forms secondary xylem on the inside and secondary phloem
on the outside, whereas the interfascicular cambium forms conjunctive tissue on the
inside and parenchymatous tissue on the outside. The interfascicular cambium
functions for sometime, and then it ceases its activity. Soon after, a new accessory
cambium ring arises by the union of the secondary parenchyma cells lying above the
conjunctive tissue and the cells of pericycle positioned outside the phloem. This first
accessory cambium ring behaves in a similar manner as of the vascular cambium,
forming secondary xylem alternating with conjunctive tissue on the inner side and
secondary phloem above secondary xylem and parenchyma above conjunctive tissue
on the outside. As a result, another ring of vascular bundles is formed which are of
secondary origin. This process may be repeated to form four or more successive
rings of vascular bundle.
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Figure: T.S. Boerhaavia stem: A. Diagrammatic; B. A part cellular
Source: Author
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Figure: Cross section of Boerhaavia stem, showing successive rings of xylem and
phloem
http://virtualplant.ru.ac.za/Main/ANATOMY/Boerh1.gif
Figure: Details of a medullary bundle, note that the xylem
contains metaxylem (MX) as well as protoxylem vessels (PX) capped by phloem (P)
http://virtualplant.ru.ac.za/Main/ANATOMY/Boerh-det1.gif
Formation of included or interxylary phloem
The groups of secondary phloem cells embedded or included in the secondary xylem
is referred to as included or interxylary phloem. They appear in the form of ‘islands’
or isolated patches apparently surrounded on all sides by xylem elements. By
definition, it is ‘the phloem that develops within secondary xylem’. These are formed
due to unusual activity of the accessory cambium. Sometimes a successive accessory
cambium is formed in the form of an arc or strip only, known as arc cambium, which
behaves unusually like the predecessor cambia and results in the formation of
included phloem. In some other instances, it suddenly changes its activity from
normal to abnormal and then again may resume back the normal activity.
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Consequently, included phloem is formed as observed in several dicot families such
as Asclepiadaceae, Nyctaginaceae, Onagraceae, Salvadoraceae, Loganiaceae and
Amaranthaceae among others. Included phloem is a characteristic feature of some
xerophytic plants and has a physiological significance. Being embedded in the xylem
tissue, they are retained and they continue to function even in the unfavourable
conditions. They serve to assimilate food for the developing buds on the restoration
of favorable conditions.
Formation of included phloem by formation of arc cambium
This is seen in stems of Achyranthes, Chenopodium, Bougainvillea, Celosia, Bosea,
Pupalia, Strychnos etc. In this case, the accessory cambium is formed which behaves
abnormally by cutting secondary xylem and sclerenchyma to the inside whereas
secondary phloem above secondary xylem and parenchyma above sclerenchyma to
the outside. After sometime, this cambium ceases its activity below the phloem. New
cambia in the form of arcs are differentiated from the parenchyma lying above the
phloem. This is called arc cambium that also behaves abnormally similar to previous
one. It also forms secondary xylem alternating with sclerenchyma inside and phloem
above the xylem and parenchyma above sclenchyma on the outside. As a result, the
earlier formed phloem gets surrounded by xylem elements and sclenchyma, which is
referred to as included or interxylary phloem. The cambium at other places continues
its normal activity to lie at the level of the arcs of cambium.
a) Achyranthes stem (Family Amaranthaceae) – The young stem bears prominent
ridges and grooves. The single layered epidermis is covered by a cuticle, followed by
hypodermis which is collenchymatous below the ridges and chlorenchymatous under
the furrows. The parenchymatous cortex is lined internally by a distinct uniseriate
endodermis. Crystals of calcium carbonate are frequently found in cells of cortex and
pith. Pericycle is multilayered with outer sclerenchyma patches. There are two welldeveloped medullary vascular bundles lying opposite to each other. In A. aspera,
these bundles fuse together and form a single amphixylic vascular bundle (with
phloem in centre surrounded by xylem on both sides). These bundles may also
remain free in the few upper internodes, but may unite lower down as seen in some
varieties. In some other species, such as A. coynei, the medullary bundles may be
four that after union become two. Fusion of vascular bundles is considered to be an
advanced feature in the line of evolution. The normal cambial ring does not develop,
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however,
accessory cambium arises in the pericycle. It acts unusually by forming
secondary xylem alternating with sclerenchyma on the inside and secondary phloem
alternating with parenchyma on the outside. Included or interxylary phloem is
formed by the formation of arcs or strips of cambium as explained above.
In some species, the activity of cambium may change after a certain period. It
may form sclerenchyma instead of secondary xylem and vice-versa. Thus can be
seen as disorganized patches of secondary xylem and sclerenchyma with constituent
included phloem. Some authors are, however, of the opinion that this is not a true
case of included phloem as it only appears to be. They argue that the phloem cells
are thin-walled and the surrounding tissue is thick-walled, which is composed of
lignified cells and are neither the tracheids nor the vessels.
b) Chenopodium stem (Family Chenopodiaceae) – In the young stem, can be seen
distinct ridges and grooves. Below the ridges is present collenchyma and in grooves
there is chlorenchyma. There is parenchymatous cortex, well-developed endodermis
and pericycle with patches of sclenchymatous cells. Vascular bundles are open,
conjoint and collateral arranged in a ring. As in Achyranthes, normal cambium ring is
not formed, but accessory cambium develops in the pericycle. This first ring of
extrastelar cambium is continuous and after producing a layer of thin walled ground
tissue forms secondary vascular bundles. The primary vascular bundles are pushed
towards the centre due to the formation of ground tissue in the beginning and they
emerge as medullary bundles. These primary vascular bundles may show secondary
growth by the activity of its constituent cambium.
The first accessory cambium behaves unusually by forming secondary xylem and
sclerenchyma on the inner side and secondary phloem and parenchyma on the outer
side. According to one view, this cambium continues to function and form secondary
vascular tissue and conjunctive tissue constantly, whereas according to another
view, it shows limited activity. In the latter case, successive rings or arcs of cambium
are produced external to the phloem forming secondary vascular bundles. These
accessory cambial rings also give rise to conjunctive tissue in which the secondary
vascular bundles get embedded in a regular or irregular pattern. In the process, it
leaves behind groups of phloem cells also embedded in the conjunctive tissue, which
can be distinguished as included phloem.
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c) Bougainvillea stem (Family Nyctaginaceae) – The structure of young stem and
formation of accessory cambial ring and its activity has been described in an earlier
written section in this chapter. However, it also shows the occurrence of included
phloem which is reported to be formed by the arc or strip of cambium formation. The
last cambium ring forms secondary xylem alternating with sclerenchymatous
conjunctive tissue towards inside, while towards outside just beneath the earlier
formed secondary phloem, it becomes functionless. Thus, arcs or strips of cambium
arise from the external parenchymatous tissue which show a similar activity as of the
accessory cambia. As a result, the secondary phloem gets enclosed by the recently
produced secondary xylem and conjunctive tissue. This gets recognized as included
or interxylary phloem.
Figure: A part of T.S. Bougainvillea spectabilis young stem
Distinguishing feature: The first ring of cambium arises from the pericycle
followed by formation of successive rings of cambia
http://www.phytoimages.siu.edu/users/paraman1/3_28_13_3/Upload28Mar13e/Bou
gainvillea1.jpg
d) Strychnos stem (Family Loganiaceae) – The stem has a cuticularised epidermis,
collenchymatous hypodermis, an outer cortex made up of chlorenchyma and an inner
parenchymatous cortex. There is a single layer of endodermis followed by 1-2
layered sclerenchymatous pericycle. The stele consists of vascular bundles arranged
in a ring. There is prominent parenchymatous pith in the centre.
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At the time of secondary growth, the normal functional cambium ring is formed.
Soon after, small segment of the cambium ring at certain points becomes inactive.
These cells may become a part of mature conducting tissue. New cambial strips or
arcs develop as accessory cambia either in the phloem or in the pericycle. These arcs
of cambium join with the edges of the normal cambial ring to form a continuous but
wavy cambial ring. This cambial ring resumes back its usual activity and hence the
groups of phloem cells are embedded in the secondary xylem. This repeatedly occurs
in other parts of the normal cambial ring and thus several patches of phloem get
engulfed in the secondary xylem.
Formation of included phloem by sudden change in activity of cambium
This is commonly seen in stem of Leptadenia, Salvadora, Combretum and Entada
etc. The accessory cambium generally cuts off secondary xylem to the inside and
secondary phloem to the outside. However, at certain points it may behave unusually
and also form secondary phloem on the inside for a considerable time before
resuming back its normal activity. This results in the formation of included or
interxylary phloem.
a) Leptadenia stem (Family Asclepiadaceae) – The young stem, circular in outline,
has an uniseriate epidermis, a few layered collenchymatous hypodermis, and a
chlorenchymatous cortex. A single layered endodermis is followed by a multilayered
pericycle with patches of sclerenchyma. Vascular bundles are open, conjoint,
bicollateral arranged in a ring. There is small parenchymatous pith in the
centre.During secondary growth, a normal cambium ring is formed by the union of
inter-and intrafascicular cambium. Initially, it shows normal activity forming
secondary xylem towards inside and secondary phloem towards outside. After
sometime, it starts functioning abnormally by cutting secondary phloem inside at
certain places in place of secondary xylem. This happens only for a short time, and
thereafter it again reverts back to its normal activity forming usual secondary xylem
inside. As a result, patches of phloem tissue get embedded in the xylem tissue. This
occurs repetitively resulting in the formation of included phloem.
The stem of Leptadenia also has another type of phloem in addition to the normal
phloem and the interxylary or included phloem. This third type is called the
intraxylary or internal (or inner) phloem. As the name suggests, this is the primary
phloem located internally to the primary xylem (thus the vascular bundles are
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treated as bicollateral). It is believed that it develops from the parenchymatous cells
of the pith or from xylem parenchyma.
Figure: T.S. Leptadenia stem (ph-phloem)
http://www.medicinalplantsarchive.us/botanical-garden/images/2601_17_13leptadenia-interxylary-phloem.jpg
Figure: T.S. Leptadenia stem to show interxylary phloem (It is the secondary
phloem included in secondary xylem; also called included phloem)
(pa-parenchyma;ph-phloem)
http://www.medicinalplantsarchive.us/botanical-garden/definition-of-anatomicalfeatures.html
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Figure: T.S. Leptadenia stem: A. Diagrammatic; B. A part cellular
Source: Author
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b) Salvadora stem (Family Salvadoraceae) – Stem when young shows a single
layered epidermis covered by a thick cuticle. Below the epidermis few layers of thinwalled parenchymatous cells constitute hypodermis. Few layered chlorenchymatous
cortex is limited by an endodermis. Below the endodermis a prominent pericyclic
zone can be seen with thick-walled fibres opposite the vascular bundles. The vascular
bundles are arranged in the form of a ring.
Salvadora persica is the representative species in which the process of secondary
growth is best described. After secondary growth, included phloem is formed due to
sudden change in activity of cambium. Just like in Leptadenia stem (described
above), here also the normal cambium at first cuts off secondary xylem and
secondary phloem on inner and outer sides, respectively. But after sometime, it
unusually forms secondary phloem inside also. After a short interval, it resumes back
its normal activity. This process occurs again and again resulting in the formation of
included or interxylary phloem.
According to another view, the cambium do not produce phloem internally
instead phloem is produced as a result of redifferentiation of parenchyma that is
produced by the cambium. At the time of secondary growth, the normal cambium
produces secondary xylem and small amounts of secondary phloem. Cambium at
certain regions begin to produce mostly parenchyma to the inside than adjacent
regions giving a wavy appearance to the xylem. The patches of parenchyma, called
as conjunctive tissue, becomes included as soon as the cambium reverts to normal.
Within the patch of conjunctive tissue, small areas become mitotically active and
differentiate into phloem. Many phloem bundles may be seen included within each
patch of conjunctive tissue. Thus, it is said that the cambium moves rapidly outward
as a ‘wave of mitotic activity’ until it reaches the outer phloem parenchyma.
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Figure: T.S. Salvadora stem: A. Diagrammatic; B. A part cellular
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Source: Author
Interxylary phloem
Intraxylary phloem
It is the secondary phloem included in
It is the the primary phloem located
secondary xylem of certain dicots.
internally to the primary xylem.
It is also known as included phloem.
It is also known as internal phloem.
It is a structural feature found in the
It is a structural feature found in the
secondary structure of the plant.
primary structure of the plant.
It is less common and found in restricted
It is more common and occurs in many
groups of plants.
families of flowering plants.
Differences between interxylary and intraxylary phloem:
Development of interxylary cork
In
some
plants
such
as
Crepis,
Artemesia
(Family
Asteraceae),
Epilobium
angustifolium, Oenothera (Family Onagraceae), Aconitum (Ranunculaceae), Sedum
(Crassulaceae), Mertensia (Boraginaceae), Geranium (Geraniaceae), Polemonium
(Polemoniaceae), Achillea fragmentissima, etc. formation of an interxylary cork, the
cork that remains within and surrounded by xylem tissue is observed. Here, the
normal cambium ring functions normally by forming normal secondary tissues. After
producing enough of secondary xylem towards inside, it has been observed that
some of the cells of xylem parenchyma dedifferentiate and become thin-walled and
non-lignified. They now function as cells of cork cambium (phellogen). These
secondary meristematic cells show normal secondary growth forming cork cells
towards outside and secondary cortical cells towards inside. This cork comes to be
termed as interxylary cork. Its main function is to provide protection during dry
season.
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Figure: A part of T.S. Epilobium angustifolium stem to show interxylary cork
https://encryptedtbn0.gstatic.com/images?q=tbn:ANd9GcQ0E3iF5v2dVPvygfjk5fBSMSjg0ft8WOCtXPq
aleSuvyPI_Ps0lg
Anomalous Secondary Growth in Monocot Stem
Monocots generally do not show secondary growth as in them, cambium is absent
and thus the vascular bundles are therefore referred to as closed. But there are
some exceptional monocotyledonous plants in which cambium develops and
secondary growth occurs, e.g. Dracaena, Yucca, Aloe, Cordyline, Dasylirion,
Baeucarnea, Sanseviera, Agave, Lomandra, Xanthorrhoea and Kingia, etc.
Dracaena (Asparagaceae) – The young stem has a thick cuticle, an epidermis
enclosing sclerenchymatous hypodermis and numerous vascular bundles scattered in
the parenchymatous ground tissue typical of a monocot stem. The vascular bundles
are closed, conjoint and collateral.Secondary growth in Dracaena is initiated by the
formation of a special meristematic zone of several cells in thickness called as
secondary thickening meristem (or thickening ring) or simply cambium. This
cambium develops in the parenchyma outside the outermost vascular bundles. The
region in which cambium appears is sometimes recognized as cortex, and at times as
pericycle. The cambium is formed in that area of the stem which ceases to elongate.
The initials of the cambium are only fusiform, arranged in a single layer and are
somewhat rectangular and tapering at the ends as seen in longitudinal section. These
are also short-lived and are continually been replaced by new dividing cells.
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The cambium acts abnormally. It gives rise to a little secondary parenchyma
towards the outside, called secondary cortex, and relatively large amount of
secondary vascular tissue along with alternating parenchyma towards the inside. The
parenchymatous tissue cells formed internally may become thick and lignified, also
known as conjunctive tissue. The internal derivatives which differentiate into
secondary vascular bundles continue to divide in various planes, and the developing
tracheary
elements
undergo
extensive
elongation.
These
secondary
vascular
bundles, embedded in conjunctive tissue, differ from the primary bundles by being
amphivasal in organization. In an amphivasal bundle, there is a centrally located
phloem tissue that is completely enclosed by peripherally arranged xylem tissue.
Around each such vascular bundle a sclerenchymatous sheath may develop. The
cambium then forms parenchyma cells inside pushing the recently formed secondary
vascular bundles towards the centre.
After some time, the cambium changes its
activity. It now forms xylem inside at those regions where earlier parenchyma was
present and parenchyma instead of xylem. As a result, a second row of secondary
vascular bundles is formed, with the bundles alternating with those of first row. This
process is repeated a number of times resulting in formation of many concentric
rings of secondary vascular bundles but differing in their position. The last one or
two rings are generally embedded in conjunctive tissue.
The cork cambium may also differentiate in peripheral tissues and function
normally.
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Figure: T.S. Dracaena stem
http://ebot.library.usyd.edu.au/view?docId=ebot/records/1671.xml;query=;brand=d
efault
A part of T.S. Dracaena stem to show amphivasal vascular bundle
http://virtualplant.ru.ac.za/Main/ANATOMY/Dracaena3.jpg
A
B
Figure A and B: A part of T.S. Cordyline mature stems showing amphivasal vascular
bundles
A.http://www.phytoimages.siu.edu/users/paraman1/3_28_13_3/Upload28Mar13e/C
orylineStemXS2.jpg
B.http://images.botany.org/set-17/17-167h.jpg
Anomalous Secondary Growth in Dicot Root
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Storage roots
In members of many dicot families, the underground roots become thickened and
get modified to become storage roots, e.g. Chenopodiaceae (Beta vulgaris),
Cruciferae (Raphanus sativus), Convolvulaceae (Ipomoea batatas), Umbelliferae
(Daucus carota), Asteraceae, etc. In them the food may be stored in the cortex or
xylem tissue or both. They generally have a relatively higher amount of parenchyma
as compared to stems. This occurs as a result of anomalous secondary growth in
these roots which is considered to be an adaptation to their storage function.
Beta vulgaris (Beet) root (Family Chenopodiaceae) – It is a diarch root showing
anomalous type of secondary growth. The primary cambium develops partly from the
pericycle cells lying opposite to the two protoxylem groups and partly from the
parenchyma cells lying between the xylem and phloem (interstitial parenchyma). It
behaves unusually by forming secondary xylem with alternating patches of
parenchyma towards inside and secondary phloem opposite to secondary xylem and
parenchyma opposite to inner parenchyma cells on the outer side. As a result, a ring
of conjoint, collateral vascular bundles is formed embedded in parenchymatous
tissue. This primary cambium soon stops functioning. A second cambium ring, which
is said to be secondary or accessory cambium, arises from the phloem parenchyma
cells or sometimes from the pericycle. This also shows anomalous activity just like
the primary cambium resulting in the formation of another ring of similar kind of
vascular bundles lying external to the first ring. Later, a series of accessory or
supernumerary/successive cambia
develop
in
the same manner
which
form
additional rings of vascular bundles. The consecutive rings of vascular bundles are
separated by wide parenchymatous tissue which serves as storage tissue.
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Figure: T.S. Beet root
http://nickrentlab.siu.edu/PLB400/images/Figure15_6.jpg
T.S. Beet young root: first stages of secondary structure formation
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http://www.phytoimages.siu.edu/users/Cusman1/3_18_12/Betavulgarisroot1.jpg
T.S. Beet root: secondary tissues generated by the first vascular cambium
http://www.phytoimages.siu.edu/users/Cusman1/3_18_12/Betavulgarisroot5.jpg
A part of T.S. Beet old root: rings of vascular bundles and parenchymatous tissue
produced by a supernumerary cambium
http://www.phytoimages.siu.edu/users/Cusman1/3_18_12/Betavulgarisroot7.jpg
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T.S. Beet old root: Four successive cambia developed
http://www.phytoimages.siu.edu/users/Cusman1/3_18_12/Betavulgarisroot4.jpg
Ipomoea batatas (sweet potato) root (Family Convolvulaceae) – Primary root is a
typical dicotyledonous root with a wide cortex, a distinct endodermis and radial
vascular bundles in pentarch or hexarch condition. The primary cambium ring
develops and functions normally producing secondary xylem towards inside and
secondary phloem towards outside. At later stages, additional anomalous cambia
arise in the form of rings around individual vascular bundle or small groups of them
from the surrounding parenchymatous cells. This secondary cambia behave
unusually by forming few sieve tube elements and laticifers, but abundant phloem
parenchyma towards outside and secondary xylem elements with a relatively large
proportion of xylem parenchyma towards inside. The vessels are clustered in the
secondary xylem and additional cambia can appear around them in the surrounding
parenchyma. This process can go on for an indefinite period. The great bulk of the
parenchymatous cells help to store reserve food material (sugar) in the root and
function as storage tissue. It leads to swelling of the root. Bark is not formed in a
large storage root of sweet potato.
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Figure: T.S. Ipomoea batatus root
http://nickrentlab.siu.edu/PLB400/images/Figure15_7.jpg
Aerial root
Tinospora cordifolia (Family Menispermaceae) – The aerial root of Tinospora is
tetrarch or pentarch. At the time of secondary growth, the cambium ring is formed in
a usual manner partly by the pericycle cells opposite the protoxylem groups and
partially by the interstitial parenchyma. Soon the differentiated wavy cambium ring
becomes more or less circular in outline. But it acts unusually by forming secondary
vascular tissues only below the primary phloem. At the region opposite to the
primary xylem (or protoxylem groups) it forms only parenchymatous tissue both
outside and inside giving rise to wide medullary rays between the secondary vascular
bundles. The secondary vascular bundles alternate with the protoxylem groups. It
consists of secondary xylem on the inside and secondary phloem on the outside
separated by a distinct cambium. The crushed primary phloem occurs as four arch-
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like patches over the secondary phloem. The primary xylem is represented by four
exarch bundles alternating with the secondary vascular bundles.
A small pith is
present in the centre. Cork cambium also differentiates simultaneously and forms its
derivatives. The secondary cortex generally bears chlorenchyma owing to the aerial
nature of root.
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Figure: T.S. Tinospora stem A. Diagrammatic; B. A part cellular
Source: Author
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Anomalous Secondary Growth
Figure: T.S. Tinospora aerial root 1. cork; 2. Pericyclic fibre; 3. Cortex; 4. Vascular
bundle
http://www.greenpharmacy.info/articles/2014/8/2/images/IntJGreenPharm_2014_
ADDITIONAL INFORMATION:
_2_105_129583_f26.jpg
ANAOMALOUS SECONDARY GROWTH IN PTERIDOPHYTES
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The structure of the vascular tissue
or stele has often been used to
separate certain groups of
pteridophytes. The simplest form
of vascular structure is
the protostele, in which a solid
vascular core or strand of tissue,
which contains xylem towards the
centre of the stele, and external to
this, a strand of phloem. In other
instances the central protostele
may contain non-vascular
parenchyma cells and this
T.S. Dicranopteris stem
http://virtualplant.ru.ac.za/Main/ANATOMY/Dicranopteris1a.gif
condition is termed a medullated
protostele or an ectophloic
siphonostele. In this definition,
a siphonostele is "any
uninterrupted stele with an
undifferentiated centre". Where
external as well as internal phloem
coexist, the structure is known as
an amphiphloic siphonostele or
sometimes equivalently,
a solenostele.
Dicranopteris is known to
contain xylem vessels, in which
the end walls are clearly perforate,
compared with the lateral wall pits
which are associated with a pit
membrane .
For additional information see:
Carlquist S and EL Schneider (2001) Vessels in ferns: structural, ecological, and evolutionary
significanceAmerican Journal of Botany. 88:1-13
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Source: http://virtualplant.ru.ac.za/Main/ANATOMY/prac5.htm
Summary

Many plants show secondary growth that is different from the normal type
which is called anomalous or abnormal or unusual secondary growth.

Anomalous or unusual secondary growth in dicot stems may occur because of
various reasons such as:
i)
Unusual position of the vascular cambium
ii) Unusual activity of the vascular cambium
iii) No development of usual cambium or if so happens, its replacement by
other accessory cambium formation and its activity
iv) Formation of included or interxylary phloem
v) Development of interxylary cork

Cambium in unusual position may be present in the form of ridges and
grooves e.g. Thinouia scandens, or may occur in the form of discrete strips,
e.g. Serjania ichthyoctona, Paullinia and Bauhunia langsdorffiana.

When cambium shows unusual activity in spite of being normal in its position,
it may behave in different manners resulting in formation of various
anomalous structures, e.g.
i)
Formation of wide medullary rays, e.g. Vitis, Clematis and Aristolochia
ii) Formation of ridges and grooves, e.g. Bauhinia
iii) Formation of phloem wedges, e.g. Bignonia
iv) Formation of irregular ‘islands’ or patches of parenchyma in xylem, e.g.
Urtica dioica

In many genera, a new cambium ring or accessory cambial rings originates in
the cortex or pericycle where either the normal cambium ring is altogether
absent or after it stops functioning. It behaves unusually resulting in the
formation of successive rings of vascular bundles embedded in parenchyma or
conjunctive tissue, e.g. Amaranthus, Mirabilis, Bougainvillea and Boerhaavia.

Sometimes, due to unusual activity of cambium, small groups of secondary
phloem cells get embedded or included in the secondary xylem which is then
called included or interxylary phloem. This may occur due to:
i)
Formation of arc cambium, e.g. Achyranthes, Chenopodium, Bougainvillea
and Strychnos
ii) Sudden change in cambium activity, e.g. Leptadenia and Salvadora
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
In some plants interxylary cork is formed that remains within and surrounded
by xylem tissue.

Though secondary growth is an exclusive feature of dicotyledonous plants,
but there are some monocots also in which cambium develops and anomalous
secondary growth occurs, e.g. Dracaena stem.

Among the dicot roots showing anomalous secondary growth, there are some
modified storage roots, e.g. Beta vulgaris, and aerial root of Tinospora
cordifolia where due to abnormal activity of the cambium, different unusual
structures are formed.
Exercise
Q.1 Fill in the blanks:
1) Phloem wedges are present in the stem of ……………………………………..
2) The vascular bundles with phloem surrounded by xylem are called
……………………………….
3) …………………………..is a monocot showing anomalous secondary growth.
4) Included phloem is also known as ……………………………………………..
5) The thick lignified parenchymatous tissue is known as ………………………..
Q.2 Give the generic name of the plant (one each) you would select to study the
following:
1) Phloem wedges
2) Fluted vascular bundles
3) Interxylary phloem
4) Medullary bundles
5) Amphivasal vascular bundles
Q.3 Match the following:
1) Fluted Vascular bundles
a. Dracaena
2) Phloem wedges
b. Bougainvillea
3) Interxylary phloem
c. Aristolochia
4) Amphivasal vascular bundles
d. Bignonia
5) Accessory cambial ring
e. Salvadora
Q.4 Name any one plant having stem with medullary vascular bundles.
Q.5 How do phloem islands arise in the xylem?
Q.6 Explain the abnormal function and position of the cambium in the following:
1) Boerhaavia stem
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Anomalous Secondary Growth
2) Beta vulgaris root
3) Leptadenia stem
Q.7 Give a concise account of the process of accessory cambium formation and its
activity describing details in one suitable example.
Q.8 Write a brief note on the secondary growth in stem of Amaranthus.
Q.9 What is interxylary cork? Enumerate giving examples.
Q.10 Write short notes on:
1) Interxylary phloem
2) Anomalous secondary growth in Dracaena
3) Accessory cambium
4) Interxylary cork
5) Medullary bundles
Q.11 What are phloem wedges? How do they arise? Explain by giving a suitable
example.
Q.12 What is unusual cambial activity? Describe with reference to the old stem of
any one of the genus: Aristolochia or Boerhaavia.
Q.13 Describe the process of anomalous secondary growth in a dicot and a monocot
stem.
Q.14 Write a detailed account of the anomalous secondary growth in Amaranthus or
Bougainvillea stem.
Q.15 Describe the process of secondary growth in Dracaena stem.
Q.16 Describe in detail the secondary growth in Boerhaavia stem and comment on
its abnormal features.
Q.17 Compare and describe the contrasting features of anomalous secondary growth
in dicot stems and monocot stems.
Q.18 With the help of well-labelled diagrams distinguish between the cross sections
of old stems of Boerhaavia and Salvadora.
Q.19 Describe the unusual anatomical features as found in Aristolochia or Bignonia
stem. Illustrate your answer with well-labelled diagrams.
Q.20 With suitable illustrations describe the unusual anatomical features in the stem
of Salvadora or Mirabilis.
Q.21 Draw well-labeled diagrams showing secondary structures of:
1) Bignonia stem
2) Dracaena stem
3) Aristolochia stem
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Anomalous Secondary Growth
4) Beta root
5) Tinospora root
Answer Key
Q.1*
1) Bignonia
2) Amphivasal vascular bundles
3) Dracaena
4) Interxylary phloem
5) Conjunctive tissue
Q.2*
1) Bignonia
2) Aristolochia
3) Salvadora
4) Boerhaavia
5) Dracaena
Q.3
1) c.
2) d.
3) e.
4) a.
5) b.
*There can be more than one correct answers for some parts; please refer
text for details.
Glossary
Amphivasal vascular bundle. Concentric vascular bundle in which the xylem
surrounds the phloem.
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Anomalous Secondary Growth
Anomalous secondary growth. A term that refers to the types of secondary
growth that are different from the normal ones or are unusual.
Bicollateral vascular bundle. A bundle having phloem on two sides of the xylem.
Bundle sheath. A layer(s) of cells surrounding a vascular bundle, which may be
parenchymatous or sclerenchymatous.
Cambium. A lateral meristem that gives rise to secondary tissue.
Chlorenchyma. The parenchyma tissue containing chloroplasts.
Closed vascular bundle. A bundle without any vascular cambium.
Collateral (vascular bundle). A bundle having phloem only on one side of the
xylem usually the abaxial side.
Concentric vascular bundle. A vascular bundle with either the phloem surrounding
the xylem (amphicribal) or the xylem surrounding the phloem (amphivasal).
Conjunctive tissue. A type of parenchymatous tissue formed between specialized
tissue; the parenchyma present between the secondary vascular bundles in
monocots.
Cork cambium/Phellogen. A lateral meristem that forms the periderm.
Cork. A layer of suberised, dead cells formed by cork cambium (or phellogen)
towards outside.
Diarch. Structural condition in a root having primary xylem with two protoxylem
groups.
Extrastelar. Occurring outside the stele.
Fascicular cambium. Vascular cambium that develops between the primary xylem
and phloem within a vascular bundle.
Included phloem. Secondary phloem included in secondary xylem of certain dicots.
Also called interxylary phloem.
Interfascicular cambium. Vascular cambium that develops in the parenchymatous
region between the primary vascular bundles.
Internal phloem. The primary phloem located internally to the primary xylem. Also
called intraxylary phloem.
Interxylary cork. The cork that develops within the xylem tissue.
Interxylary phloem. See Included phloem.
Intraxylary phloem. See Internal phloem.
Medullary bundles. Vascular bundles located in the pith region.
Open vascular bundle. Vascular bundle having a vascular cambium and thus being
potential for secondary growth.
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Anomalous Secondary Growth
Phellogen. See Cork cambium.
Secondary growth. Growth resulting from the activity of the vascular cambium and
cork cambium; results in an increase in axis diameter.
Successive cambia. Vascular cambium originating in phloem or pericycle outside
the regularly formed vascular cambium. Characteristic of some plants with
anomalous type of secondary growth.
Tetrarch. Structural condition in a root having xylem with four protoxylem groups.
Vascular bundle. A strand-like part of the vascular system composed of xylem and
phloem that extends throughout the plant body.
Vascular cambium. See Cambium.
Xylary. Pertaining to xylem.
Further Reading
Beck, C.B. (2010). An Introduction to Plant Structure and Development: Plant
Anatomy for the Twenty-First Century. Second Edition. Cambridge University
Press, New York, USA.
Cutter, E.G. (1971). Plant Anatomy: Experiment and Interpretation. Part2: Organs.
Edward Arnold, London.
Dickison, W.C. (2000). Integrative Plant Anatomy. Harcourt Academic Press, USA.
Esau, K. (1977). Anatomy of Seed Plants. John Wiley & Sons, Inc., Delhi.
Fahn, A. (1974). Plant Anatomy. Pergmon Press, USA.
Mauseth, J.D. (1988). Plant Anatomy. The Benjammin/Cummings Publisher, USA.
Metcalfe, C.R. and Chalk, L. (1983). Anatomy of the Dicotyledons. 2nd ed. Clarendon
Press, Oxford.
Pandey, S.N. and Chadha A. (1996). Plant anatomy and Embryology. Vikas
Publishing House Pvt. Ltd., New Delhi.
Web links
http://virtual plant.ru.ac.za/Main/ANATOMY/prac5.htm
http://www.slideshare.net/jasperidium/anomalous-secondary-growth
https://www.youtube.com/watch?v=879v7ioN
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