Secondary Growth in Stems: Wood, Bark and Surface Features - 1
Secondary Growth in Stems
Secondary growth in plants is responsible for the increase in girth or diameter of the
plant by the addition of secondary vascular tissue and periderm. All woody
plants exhibit extensive secondary growth, but many herbaceous plants have some
secondary growth.
Secondary growth has commercial value (wood and wood products) for humans and
dimensional value for plant, because secondary growth allows for much greater size
and volume.
Although we focus on secondary growth in stems, roots, too, have secondary growth
patterns that parallel the secondary growth of stems. Leaves have minimal, if any,
secondary growth, generally is restricted to strengthening vein tissue.
Secondary growth originates from two lateral meristems: vascular cambium,
derived from procambium retained in vascular bundles during primary growth, and
cork cambium, produced by dedifferentiation of cells in the outer cortex.
• Vascular cambium produces secondary xylem to the interior of the cambium
layer, and secondary phloem exterior to the cambium layer.
• Cork cambium produces the secondary dermal tissue, called periderm,
comprised of cork, cork parenchyma and cork cambium.
The secondary dermal tissue and secondary phloem form bark. Wood is comprised
of secondary xylem. Vascular cambium forms a layer between bark and wood.
Initiation of Secondary Growth
Secondary vascular tissue is initiated in the primary growth stem from cambium
cells (initials) located in open vascular bundles. Most of the cambium produces cells
that are vertically elongated and form the axial growth of the stem. Other cambium
cells are oriented laterally and form rays that radiate out towards the surface of the
stem. (Naturally these two types of vascular cambium have special names: fusiform
initials and ray initials.)
In addition, most cambium initials divide in a plane so that cell plate formation is
parallel to the surface of the stem (periclinal divisions), producing xylem or phloem
cells. Some cambium cells divide in a plane perpendicular to the cell surface,
producing more cambium to accommodate to the increasing girth of the plant
(anticlinal divisions).
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Cambium initials
Anticlinal (CÆ C) and Periclinal (CÆ X or P) divisions
In the transition from primary to secondary growth in those stems that have discrete
vascular bundles, the early vascular cambium produces meristematic cells both within
the vascular bundle, called vascular bundle cambium (fascicular cambium), and in
the pith rays between adjacent vascular bundles (inter-fascicular cambium) t o
produce a complete ring of vascular cambium.
Transition to Secondary Growth
Once a vascular cylinder is formed, cambium produces secondary xylem toward the
interior of the stem and secondary phloem toward the exterior of the stem.
Additional cambium cells are found between the xylem and phloem, and also divide
"sideways" to maintain a continual cambium cylinder as the diameter of the stem
increases. Most cells produced are xylem.
Cambium exhibits seasonal dormancy in areas that have distinct seasons, such as the
temperate biomes. This contributes to the growth rings common in wood. In many
tropical species cambium is always active, and wood lacks distinctive rings.
Secondary Growth in Stems: Wood, Bark and Surface Features - 3
Sambucus: Early cambium activity
Sambucus: End of First Year of Secondary Growth
Tilia: One-year stem cross section
Section of 3-year wood and bark
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Secondary dermal tissue is produced from the cork cambium, which produces cork
tissue and cork parenchyma cells. The tissues produced by the cork cambium are
collectively called the periderm.
As the original epidermis and cortex layers are destroyed and sloughed off, they are
replaced by cork. Primary phloem will also be replaced with the expansion in girth as
the plant grows. Cork tissue interlaces with secondary phloem tissue to form bark.
The continued production of new vascular tissue (xylem and phloem) forces the stem
to expand outward. Older phloem and cork are eventually sloughed off, and continue
to be replaced with more bark. The alternating phloem and cork tissues can often be
distinguished by the layers of phloem fibers. This creates a number of interesting
patterns in bark. The bark pattern of a tree is also a species characteristic. In
contrast, all of the secondary xylem is retained as the stem expands, forming the
wood portion of the stem.
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Tissues in Secondary Growth
Xylem - wood
Cells that mature inward from the vascular cambium are xylem tissue. This tissue
forms the part of the stem we call wood. Wood forms the bulk of secondary growth,
and, as we know, can have a great volume. Most dicots have some secondary growth
in vascular tissue. All woody plants (shrubs and trees) are perennials with significant
secondary growth. All conifers are woody plants. Some monocots are “woody”, but
they have very special ways of obtaining strength and dimension and will be discussed
at the end of this section. In general, wood is comprised of:
• Xylem vessels and/or tracheids
• Fibers
• Ray parenchyma, which is responsible for lateral conduction of water.
Dicot Wood
Dicot wood is comprised of vessels, fiber and parenchyma rays. The organization of
xylem tissue in wood will be discussed a bit later when we look at wood patterns and
orientation of wood, features that most humans have, on occasion, observed.
Conifer Wood
Much of the wood used commercially is from conifers. Conifers are often called
softwood, while the wood of flowering plants (dicots) is known as hardwood. These
designations have no meaning for the strength of wood.
Conifer wood contains only tracheids, no vessels and minimal parenchyma, so conifer
wood appears more uniform than the wood of flowering plants that have larger
diameter vessels, along with the numerous fibers. Conifer tracheids are characterized
by prominent bordered pits along their walls. Microscopically, the bordered pits of
tracheids are spectacular. In addition, resin ducts (or canals) are present in
conifers. A resin canal is lined by a ring of parenchyma cells. Resin is believed to be a
defense mechanism that helps protect the plant from predator damage.
Pine: two-year cross section
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Wood (Xylem) Features
When wood is observed, many different patterns are possible depending on the
orientation (or plane) of the wood when the wood is cut, as well as differential pattern
of growth rings found in most wood. Since we know that cells have three dimensions,
the way in which tissue is cut and observed will give a different appearance to the
cells. These patterns are referred to as the "grain" of the wood.
The orientation of the rays is helpful in determining grain patterns. Although we can
often distinguish wood patterns macroscopically, looking at the cells microscopically is
more revealing. Inspecting wood for grain pattern is easier at first in Conifers than in
Angiosperms because the tracheids in conifer wood are more uniform.
Xylem patterns and wood grain
Wood is commonly cut in one of three ways: transverse, tangential
or radial
Transverse or cross section
A transverse section will cut wood so that we see the "ends", or cross sections (short
dimension) of vessels, tracheids and fibers and the tops (long dimension) of rays.
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Radial longitudinal section (quarter-sawed timber)
A radial section of wood will be cut along the radius of the stem. The long sides of
vessels, fibers or tracheids will be visible. The sides of rays (note how rays stack up)
will be visible as “streaks running at right angles across the lengthwise vessels and
tracheids. A radial section of wood is cut parallel to the ray direction. In conifers one
sees the “face” view of the conspicuous bordered pits of the tracheids.
Tangential longitudinal section (plain-sawed timber)
A tangential section of wood is cut perpendicular to the radius of the stem. Again,
the long sides of vessels, fibers or tracheids will be visible. However, the rays will be
seen in cross section, so you will see the ends of the rays.
Pine wood: transverse section
radial section
tangential section
Growth Rings
The cambium layers in woody secondary growth plants of temperate biomes have
seasonal dormancy. Growth is most active in spring and tapers in summer, ceasing in
fall. Spring (or early) wood typically has larger vessels that are more porous and
fewer, smaller rays. Summer (or late) wood is comprised of denser, smaller cells
with thicker walls. The first cells of the next season’s spring wood form are produced
next to the smallest summer wood cells of the previous year. Annual growth rings are
the result of the alternating pattern of spring and summer wood.
Vessel size in spring and summer wood
Section of Bristlecone Pine Wood 4240 Æ 4210 BC
Secondary Growth in Stems: Wood, Bark and Surface Features - 8
Vessel distribution and vessel size in wood are other features that lend “character”
to wood.
• Ring porous wood has an abrupt transition between spring and summer wood.
Spring vessels are distinctively large and fewer in number, and the summer
vessels are numerous and small.
• The vessels in diffuse porous wood are produced more uniformly in both
spring and summer wood.
Ring porous wood
Diffuse porous wood
Reaction Wood
Growth rings can be uneven if the stem has an obstacle, such as a huge rock on one
side that inhibits expansion, or if the stem is leaning in one direction. Compression
wood forms when there is more growth in the lower side to compensate and
straighten the stem. Tension wood forms when there is more growth on the upper
side of the bend to "pull" the stem upright.
Reaction Wood
Many trees native to tropical regions that have a uniform climate, such as the tropical
rain forests, may not have annual growth rings, and have a fairly uniform grain.
Tropical trees that grow in areas with seasonal climates do have growth rings,
although the seasons may be wet/dry rather than cold/warm
Secondary Growth in Stems: Wood, Bark and Surface Features - 9
The pattern of rays may also affect wood appearance. Rays that form in clumps
may be detectable without magnification, whereas rays that are a single cell wide are
visible only microscopically.
Another difference in wood appearance is caused by aging of wood. Heartwood
contains the older xylem that is non-functioning. Heartwood can rot away, leaving a
hollow core, with living tree around it. Heartwood is typically darkened in appearance,
caused, in part, by the accumulation of materials. The functioning xylem toward the
exterior is the sapwood.
Wood density is an important feature in the use of wood commercially as is its
specific gravity and moisture content. Wood with uneven growth rings has different
“drying” characteristics than wood with more uniform growth. This affects its
commercial use.
Knots are caused by branches that originated when the stem was younger and
subsequently died. Branches formed lower on the stem often die as the tree
increases in height. The eventual growth in stem girth surrounds the dead branch,
forming a knot. Knots can be tight (if there is a cambium connection between the
branch and trunk or loose if there was bark surrounding the branch.
Deciduous is another term used with trees. Deciduous refers to a seasonal loss of all
of a tree's leaves. Some trees are evergreen, which means that individual leaves are
replaced as they die.
Secondary Growth in Stems: Wood, Bark and Surface Features - 1 0
Bark
Although the bulk of secondary growth occurs in the xylem (wood), there is a second
major area of secondary growth, the bark. The bark comprises all regions of the
secondary growth stem exterior to the cambium. This includes phloem from the
vascular cambium, and periderm (cork) tissues.
Cork (Periderm)
Primary growth produces the epidermis for protection and collenchyma cells for
strength. However, as the stem enlarges with secondary growth, the primary
tissues cannot grow, and must be replaced. Cork cambium, which originates
from parenchyma-like cells of the outer cortex (or rarely epidermis cells),
produces the cork tissue. Cork is also called phellem and the cork cambium is
also called phellogen. The cork, cork cambium and cork parenchyma are
collectively called the periderm. For reasons unknown, someone fond of terms
provided duplicate names for all components of the secondary dermal tissues,
all starting with the letter “P”.
Characteristics of cork
• Several layers of thin walled and flattened cells that die at maturity
• Walls have suberin (waxy fat impermeable to water; the same chemical
found in the casparian strips of the root endodermis layer, and in the
abscission zone in leaves. Walls may also contain lignin.)
• Provides mechanical protection to stem
• There may be a region of parenchyma cells produced interior to the cork
cambium, too. The cork parenchyma cells can be called phelloderm
(another term that can be ignored).
Early cork
Lenticels
Lenticels in Bark
Cork is generally impervious to fluids and gases so that special structures for
gas exchange are required to provide oxygen to the living cells of the secondary
growth region. Lenticels are weak "eruptions" of parenchyma cells through
which gases can diffuse. Lenticels also contribute to the appearance of bark
Secondary Growth in Stems: Wood, Bark and Surface Features - 1 1
Phloem
Secondary phloem originates from vascular cambium cells that divide and
specialize outward. As secondary growth progresses, functioning phloem sieve
tubes, companion cells and phloem fibers are spread out in patches
interspersed with dilated phloem rays of parenchyma cells. Expansion nearer
the exterior of the stem requires a greater surface area. The phloem rays are
large parenchyma cells that can fill more space so that the phloem region can
keep up with the expanding circumference of the stem. The phloem rays often
connect to xylem rays. Since the phloem consists of thin-walled sieve tubes
and their companion cells, the older phloem gets crushed and compacted with
the lateral expansion of the stem. New cork cambium tissue is formed from
non-functioning phloem parenchyma and forms new periderm layers to separate
older phloem from newer phloem. The older phloem and periderm layers are
eventually sloughed off.
Dilated phloem rays
Phloem
Older Phloem and Cork
Secondary Growth in Stems: Wood, Bark and Surface Features - 1 2
Aging of Bark
Since volume expands constantly the bark must likewise increase in girth to
accommodate the interior expansion. Old bark is continuously being pushed
outward and on occasion will be shed from tree by sloughing off. The different
ways of sloughing result in unique bark patterns, such as papery, shaggy or
scaly or furrowed.
Papery bark
Shaggy bark
Scaly bark
Furrowed bark
Secondary Growth in Stems: Wood, Bark and Surface Features - 1 3
Secondary Growth in Monocots
As mentioned previously, monocots generally lack secondary growth and most
monocots are small and herbaceous (which is consistent with the lack of secondary
growth). However there are some significant exceptions to the overall small size of
monocots and there are a number of ways monocots increase in girth with little or no
secondary growth.
•
Plant increases in diameter as the seedling emerges by producing a thickened
meristem that produces a wide procambium region. This results in a large diameter
base with many strong vascular bundles and much vascular parenchyma. The
apical meristem containing many leaf primordia is essentially embedded into the
thickened meristem. The unique meristem cap produces stem growth that has:
• Uniform upward diameter
• Thickened parenchyma cells for support
• Long lived phloem
• Large apex (or tip) providing for
o Leaves with large vascular connections that sheath the stem
increasing the diameter of the axis
Example = Palms
Thickened palm meristem
•
•
•
Uniform stem diameter in palm
Prop roots develop providing long-term support for the plant
Example = Pandanus or corn
Produce a cambium that produces additional vascular bundles, but not a "wood".
This increases volume but is not necessarily a "tree-like" organism
Example = Agave
Sheath the stem with giant leaves that have extraordinary vascular tissue (vein)
connections. Veins, the vascular tissue of leaves, have many sclerenchyma fibers.
Often such plants are quite short-lived, although they achieve big dimensions
Example = Banana
Secondary Growth in Stems: Wood, Bark and Surface Features - 1 4
External Features of Stems
Before we leave our discussion of secondary growth and stems, we should spend a bit
of time discussing the external features of twigs – young wood stems and branches.
There are a number of easily recognized common surface features of the twigs of any
woody stem, particularly when dormant. In fact, for deciduous trees, there are
identification keys that focus on winter twig and bud patterns.
Most twigs have a terminal (or apical) bud, located at the tip of the twig, as well as
a number of axillary (or lateral) buds, located in the axils of leaves. Dormant buds
are protected by one to several bud scales, which are modified leaves. Leaves are
attached to the stem at nodes; the space along the stem between leaves is called an
internode. Nodes and internodes are distinctive on twigs with or without their leaves
attached.
A leaf scar remains on the stem when leaves dehisce. The pattern of leaf scars is a
species characteristic. The vascular bundles of the leaf petiole also leave bundle
scars within the leaf scar. Each growing season, a ring of bud scale scars that
protected the dormant terminal bud can be seen on twigs. This is one way to
determine the age of a twig or branch. Lenticels are also more visible on twigs than
on older stems where the bark patterns may mask them.
As secondary growth continues, and branches increase in girth, the expansion of cork
and bark eventually replaces the features found on twigs. Older branches take on the
bark pattern associated with the specific species.
Secondary Growth in Stems: Wood, Bark and Surface Features - 1 5
Some Examples of Winter Twigs