Chapter 8
STRUCTURE OF WOODY PLANTS
NOTES MADE FROM MAUSETH AND RAVEN.
In many plants - most monocots and some eudicots like Ranunculus - growth ends with the maturation
of the primary tissues.
Gymnosperms, magnoliids and most eudicots continue increase in diameter in regions that have
stopped elongating as a result of the activity of the vascular cambium and the cork cambium.
ANNUALS, BIENNIALS AND PERENNIALS.
In annuals the entire life cycle, from seed to flower and seed again, occurs in one growing season.
Only the dormant seed bridges one cycle to the next.
In biennials, two seasons are needed to complete the cycle: the first growing season is for the
maturation of the root, stem and leaves, and second growing season is for flowering, fruiting and seed
formation. Death ends the cycle.
Perennials are plants that persist for several to many years.
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Herbaceous eudicots survive underground during the unfavorable season.
Woody eudicots and magnoliids survive above ground but usually stop growing during the
unfavorable season.
Woody perennials flower only when they are adults, which may take many years.
VASCULAR CAMBIUM
The vascular cambium is responsible for the secondary growth of the plant body.
The vascular cambium arises from the procambium that remains undifferentiated between the primary
xylem and primary phloem, and from parenchyma that occurs in the interfascicular region, which
occur between the vascular bundles or fascicles. See Fig. 8-3.
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Fascicular cambium is found within the vascular bundles
Interfascicular cambium arises in the parenchymatous regions between vascular bundles.
More xylem than phloem is produced during secondary growth.
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In elderberry, the primary phloem becomes obliterated and replaced by fibers (sclerenchyma).
In basswood, the rays in the secondary phloem become dilated in order to adjust to the increase
in girth.
The vascular cambia and secondary tissues of root and stem are continuous with one another. There is
no transition region in the secondary plant body and there is in the primary plant body.
Vascular cambium cells are highly vacuolated. They exist in two forms:
1. Fusiform initials: longer than wider.
2. Ray initials: horizontally oriented, slightly elongated or squarish.
Periclinal divisions of the initials and their immediate derivatives produce secondary xylem and
secondary phloem.
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In periclinal divisions, the cell plate develops parallel to the axis or surface of the stem or root.
Cells appear one behind the other along the radius of the stem.
Derivatives to the outside develop into phloem; those to the inside develop into xylem.
Cells produced by the fusiform initials are oriented vertically forming the axial system.
Ray initials produced horizontal cells, which form the vascular rays or the radial system.
The vascular rays are composed of parenchyma and variable in length.
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They move food from the secondary phloem to the secondary xylem and water from the
secondary xylem to the secondary phloem.
Vascular rays also serve as storage center for starch, proteins and lipids.
Rays may also synthesize some secondary metabolites.
It is often difficult to distinguish between the cambial initials and their immediate derivatives, which
may remain meristematic for some time before differentiating.
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Vascular cambium may be used for the initials only or for the initials and their derivatives.
Cambial zone is also used to refer to the initials and their undifferentiated derivatives.
Anticlinal division of initials adds cells to the side in order to accommodate the increase in
circumference due to the addition of xylem to the inside of the cambial zone.
The vascular cambium enters a dormant period during winter in temperate zones.
The expansion of the buds and resumption of their growth trigger reactivation of the cambium.
Developing shoots produce auxin, a plant hormone that travels down the stem and stimulates cambium
to divide.
In the tropics, the cambium of many trees remains active throughout the year. This number varies
greatly between the different tropical regions of the world.
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75% remain active throughout the year in India.
43% in the Amazon basin.
15% in Malaysia.
FUSIFORM INITIALS
1. Anticlinal division increases the diameter of the stem.
2. Periclinal division produces a derivative (daughter) cell and an initial cell.
Periclinal divisions of the initials and their immediate derivatives produce secondary xylem and
secondary phloem.
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In periclinal divisions, the cell plate develops parallel to the axis or surface of the stem or root.
Cells appear one behind the other along the radius of the stem.
Derivatives to the outside develop into phloem; those to the inside develop into xylem.
Periclinal division produces two cell: one continues to be a fusiform initial; the other differentiates
into a cell of secondary xylem or secondary phloem.
Cells produced by the fusiform initials are oriented vertically forming the axial system.
Anticlinal division of initials adds cells to the side in order to accommodate the increase in
circumference due to the addition of xylem to the inside of the cambial zone.
The vascular cambium enters a dormant period during winter in temperate zones.
The expansion of the buds and resumption of their growth trigger reactivation of the cambium.
Developing shoots produce auxin, a plant hormone that travels down the stem and stimulates cambium
to divide.
In the tropics, the cambium of many trees remains active throughout the year. This number varies
greatly between the different tropical regions of the world.
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75% remain active throughout the year in India.
43% in the Amazon basin.
15% in Malaysia.
Ray Initials
Ray initials produced horizontal cells, which form the vascular rays or the radial system.
The vascular rays are composed of parenchyma and variable in length.
Tracheids may be present in some species.
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Rays run parallel to the radius of the stem.
They move food from the secondary phloem to the secondary xylem and water from the
secondary xylem to the secondary phloem.
Vascular rays also serve as storage center for starch, proteins and lipids.
Rays may also synthesize some secondary metabolites.
Rays may be uniseriate (one cell wide) or multiseriate (many cells wide).
It is often difficult to distinguish between the cambial initials and their immediate derivatives, which
may remain meristematic for some time before differentiating.
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Vascular cambium may be used for the initials only or for the initials and their derivatives.
Cambial zone is also used to refer to the initials and their undifferentiated derivatives.
Arrangement of Cambial Cells
Fusiform initials may occur in regular horizontal rows, called storied cambium or irregularly, without
horizontal pattern, nonstoried cambium.
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Storied cambium: the ends of the fusiform initials appear to be at about the same level.
Nonstoried cambium: the end of the fusiform initials overlap with the adjacent initials.
SECONDARY XYLEM
Cells produced by the fusiform initials are oriented vertically forming the axial system.
Ray initials produced horizontal cells, which form the vascular rays or the radial system.
Wood is secondary xylem.
Woods are classified as either hardwood or softwood.
Hardwoods are angiosperms (magnoliids and eudicots), and softwoods are conifer wood.
This classification does not necessarily agree with the density (weight per unit of volume) and
hardness of the wood.
Fusiform cells can differentiate into xylem parenchyma. This axial parenchyma is important as a
temporary reservoir of water. Many desert-adapted trees have abundant xylem parenchyma.
Fibers provide maximum strength if grouped together in masses, which is their usual arrangement. If
fibers are located around a vessel, their secondary walls reinforce the walls of the vessel and help it
resist collapse.
The radial system is simple. In woody angiosperms, it contains only parenchyma, arranged in
uniseriate, biseriate or multiseriate groups of cells. Ray parenchyma store carbohydrates ad other
nutrients during dormant periods ad conduct material radially for short distances.
Wood sections: See figures on pages 168-173.
 Transverse sections are cross sections, cut at right angles to the long axis of the root or stem.
 Radial sections are cut parallel to the radius and parallel to the rays.
 Tangential sections are cut at right angles to the rays.
1.
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Conifers wood:
Lack vessels.
Seldom with a small amount of wood parenchyma and fibers.
In some conifers, the only wood parenchyma is those cells associated with resin ducts.
Resin ducts are large intercellular spaces lined with parenchyma cells that secrete resin into the
duct.
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Long tapering tracheids are the major component.
Tracheids have large, circular bordered pits on their radial walls.
Pits form pit pairs with adjacent tracheids.
Pits are characterized by having a torus, a thickened central portion of the pit membrane.
The torus is slightly larger than the apertures and may block the flow of water between tracheids.
The pit membrane or margo is very porous.
Rays are made of parenchyma and tracheids.
Rays are usually one cell wide and from one to 20 cells high.
Rays make about 8% of the wood.
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Angiosperm wood.
Have vessels, tracheids, fibers and parenchyma.
Axial parenchyma acts as temporary reservoir of water.
Rays are larger than those of gymnosperms ranging from one to several cells wide up to several
hundred cells high.
On the average, rays make about 17% of the volume of the wood.
Dicot rays lack tracheids; they are made of parenchyma cells only.
Large vessel elements tend to displace rays.
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Ray parenchyma stores water and nutrients.
There are two basic types of ray parenchyma: upright cells and procumbent cells. See Fig. 8.15.
Plasmodesmata may occur between upright ray parenchyma and axial parenchyma if the cells are
adjacent to each other.
Nutrients in the upright ray parenchyma can pass to adjacent axial tracheids and vessels through the
thin membrane of the upright cells and the pits in the tracheary elements.
Growth rings
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Growth rings result from the periodic activity of the vascular cambium.
If a ring represents one season's growth it is called an annual ring.
Availability of water and other environmental factors may cause more than one ring in a
season.
Trees that have continuous cambial activity like those in the tropics lack growth rings.
Environmental factors will affect the width of rings: light, rainfall and temperature.
Differential growth of spring or early cells and autumn or late cells forms the rings.
Some angiosperms produce larger vessels early in the season than those late in the season.
These trees produce ring-porous wood.
Other angiosperms produce vessels of uniform size and distribution. These are called diffuseporous wood.
Wood produced in the spring is called early or springwood; wood produced in the summer is
called summer or late wood.
Heartwood and sapwood.
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As the wood becomes older, it gradually becomes non-functional in conduction and storage.
Old wood becomes infiltrated with oils, gums, resins and tannins.
Old wood is often darker and is called heartwood.
Younger wood is lighter in color and is called sapwood.
Sapwood is the functional portion of the secondary xylem.
Some trees do not have a clear distinction between sapwood and heartwood.
Tyloses are formed in vessels when they become non-functional.
Tyloses are balloon-like outgrowths from rays or axial parenchyma cells.
Tyloses help to change sapwood into heartwood by dropping into old vessels and mixing with
gums, phenolics and other aromatic substances to clog the vessels.
Tyloses have been found in a number of pine species.
Reaction wood.
Newly formed plant parts like branches produce tissues that counter act gravity action so the plant
parts are placed in the best possible position for photosynthesis.
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Compression wood is produced by the increase activity of the cambium on the lower side of the
bent stem and results in eccentric rings.
It is a response by a leaning branch or stem to counteract the force of gravity.
Compression wood forms in gymnosperms.
It contains large amount of lignin and intercellular spaces.
It causes the limb to straighten by pushing and expanding the limb upwards.
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Tension wood is produced on the upper side of the bent stem of angiosperms.
Tension wood has many gelatinous fibers made of cellulose and few vessels.
The gelatinous fibers shrink and pull the stem upright.
SECONDARY PHLOEM
There is primary and secondary phloem.
The first formed primary phloem, the protophloem, is often stretched and destroyed during elongation
of the organ.
Two types of sieve elements are recognized: sieve cells and sieve tube elements.
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Sieve cells are found only in gymnosperms.
Sieve tube cells are found only in angiosperms.
Sieve elements are variable in seedless vascular plants and are simply called sieve elements.
Parenchyma cells are also found in the phloem and are associated with the storage of a variety of
substances.
Fibers and sclereids may also be present in the phloem and help in supporting the plant body.
More xylem than phloem is produced during secondary growth.
In elderberry, the primary phloem becomes obliterated and replaced by fibers (sclerenchyma).
In basswood, the rays in the secondary phloem become dilated in order to adjust to the increase in
girth.
OUTER BARK
In most woody stems and root, periderm formation follows the beginning of secondary growth in the
xylem and phloem.
The addition of secondary xylem and phloem pushes the cortex and epidermis outward. This requires
that tissues in the periphery of the plant either grow in circumference or be torn apart.
The periderm replaces the epidermis as the protective covering on the secondary plant body.
The periderm consists of three layers:
 Cork cambium of phellogen.
 Cork or phellem; this is the protective tissue formed to the outside of the phellogen.
 Cork parenchyma or phelloderm, the living parenchyma formed to the inside of the phellogen.
The periderm first appears during the first year of growth usually in the cortex right below the
epidermis.
In some plant, it appears in the primary phloem and in other, in the epidermis.
During differentiation, cork cell inner walls become lined with layers of suberin and wax that make the
cell wall highly impermeable to water.
Cork cells are dead at maturity.
Phelloderm cells are living at maturity and resemble cortical parenchyma cells.
At the end of the first year of growth, the following tissues are present in the stem:
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Remnants of the epidermis
Periderm
Cortex primary phloem
Secondary phloem
Vascular cambium, secondary xylem
Primary xylem
Pith
The bark includes all tissues outside the vascular cambium that is phloem, cortex and periderm.
At the end of the first year growth, the bark includes any primary tissues still present, the secondary
phloem, the periderm, and any dead tissues remaining outside the periderm.
Each year the vascular cambium adds secondary phloem to the outside area.
Usually less secondary phloem than secondary xylem is formed.
The soft-walled cells of sieve tube members, companion cells and parenchyma are commonly crushed.
Eventually, new periderm appears in between the old crushed secondary phloem and newer secondary
phloem
The old phloem is separated from the newer phloem and is sloughed. This results in very little
accumulation of secondary phloem.
As the stem or root increases in girth, parenchyma cells of the cortex and rays divide and enlarge. In
this manner, the old secondary phloem keeps up for a while with the increase in circumference of the
stem or root.
The first cork cambium may remain active for many years (20 years) as in apples and pears.
In most woody roots and stems, additional periderms are formed as the axis increases in
circumference.
New periderm is formed deeper in the bark from phloem parenchyma cells that become meristematic
and are no longer active in transporting materials in the stem.
The innermost cork cambium separates the living inner bark from the dead outer bark.
With maturation of the suberized cork cells, the tissues outside them are separated from the supply of
water and nutrients. Hence, the outer bark consists entirely of dead tissues.
The inner bark consists of all tissues inside the innermost cork cambium and extends to the vascular
cambium.
In some barks, the newly formed periderms develop as discontinuous overlapping layers, resulting in
formation of a scale type of bark called scale bark, like in young pines and sycamore.
In other trees, the newly formed periderms arise, as more or less continuous, concentric rings around
the axis, resulting in formation of a ring bark, like grape and honeysuckle.
The barks of many plants are intermediate between ring and scale barks.
In most trees the only active phloem is the one produced in the early spring. This phloem dies at the
end of the growing season in the autumn.
The part of the inner bark that is engaged in the transport of food is called functional or conducting
phloem.
Phloem parenchyma and ray parenchyma cells may continue to live and function even if the old
phloem cells are dead. This mixture of living parenchyma and dead phloem is called the
nonfunctional or nonconducting phloem.
Lenticels and Oxygen Diffusion
Lenticels allow gas exchange through the periderm.
The suberized cork cells are compactly arranged and present an impermeable barrier to water and
gases.
They form during the development of the first periderm normally below a stoma or group of stomata.
Lenticels are portions of the periderm with many air spaces in between the cells and may appear as
raised round, oval or elongated areas.
In older portions of the stem, lenticels form at the bottom of cracks in the bark where new periderm is
being formed.
Initiation of Cork Cambia
In some species the first cork cambium arises before the twig or root is one year old. On stems the
color of the surface often changes from green to tan.
In other species, cork begins to form only after the stem is several years old.
Delayed formation of bark is common in plants that depend on cortex chlorenchyma
The first cork cambium may arise in the epidermis, cortex, primary phloem or secondary phloem.
Subsequent cork cambia may form shortly afterward, sometimes in the same season. If the growth in
diameter is slow, new cork cambia may arise in as much as 10 years.
These later cork cambia usually form deep in the secondary phloem.
The first bark on young stems usually differs from bark formed when the stem is older.
If the first cork cambium arises in…
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Epidermis, it contains only periderm and cuticle and is very smooth.
Cortex, the first outer bark contains periderm, cortex and epidermis; this is smooth and contains
any cortical secretory cells that were present.
Later cork cambium arises in the secondary phloem; this outer bark contains only cork and phloem.
SECONDARY GROWTH IN ROOTS
The roots of monocots lack secondary growth and consist only of primary tissue.
The roots of many herbaceous eudicots undergo little or no secondary growth and remain mostly
primary in composition.
Secondary growth in roots and stems consists on the formation of
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Secondary vascular xylem and phloem from the vascular cambium.
Periderm composed mostly of cork tissue, from the cork cambium.
Secondary growth begins in portions of the root that are not elongating, in the area between the xylem
and phloem.
Pericycle cells also begin to divide at same time in the area opposite to the protoxylem
Eventually the cambium becomes circular due to the accumulation of secondary xylem toward the
center of the vascular cylinder.
Repeated divisions towards the inside and outside add secondary xylem and secondary phloem added
to the root.
With increases divisions toward the inside secondary xylem and secondary phloem are added to the
root.
Files of parenchyma cells that extend radially form rays in the secondary xylem.
With increases in width of the secondary xylem and phloem, most of the primary phloem is crushed
and obliterated.
The periderm replaces the epidermis in most woody roots.
Division of the pericycle layer causes an increase of pericycle layers in a radial extent.
The formation of the periderm follows the initiation of secondary xylem and phloem production.
As the pericycle cells are pushed out by the addition of secondary xylem and phloem, they multiply
and begin to produce more pericycle cells.
The cork cambium arises in the outer part of the pericycle.
Activity of the cork cambium gives rise to cork cells to the outside and to phelloderm toward the
inner surface.
Collectively, these three tissues, cork, cork cambium and phelloderm, form the periderm.
The remaining pericycle develops into a cortex tissue.
With the formation of the periderm, the epidermis, the cortex and the endodermis eventually die and
are sloughed off.
At the end of the first year’s growth, the following tissues are present in a woody root from outside to
inside: possible remnants of the epidermis and cortex, periderm, pericycle, primary phloem (fibers if
present and crushed soft-walled cells), secondary phloem vascular cambium, secondary xylem, and
primary xylem.
ANOMALOUS FORMS OF GROWTH
Anomalous secondary growth is produce by cambial activity of cells that differ in their location in the
stem.
In other storage roots like beets, the cambium form concentric rings with the production of little
secondary xylem to the inside and phloem to the outside of the cambium, but it forms storage
parenchyma in both directions.
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In carrots, Daucus carota, the root develops like in other non-fleshy roots except that there is a
large amount of parenchyma cell in the secondary xylem and phloem.
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Sweet potato, Ipomoea batatas, has a normal cambium and accessory cambia in the roots that
forms around some vessel.
o These additional cambia produce a few tracheary cells toward the vessels and a few sieve
tubes away from them, mainly to produce storage parenchyma away from them.
o Most of the xylem and phloem is made of storage parenchyma.
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Included phloem. In several eudicots, a vascular cambium of the common type arises and
produces ordinary secondary xylem and phloem. After a short period, the cambium cells stop
dividing and differentiate into xylem. There is no longer cambium. Cells in outermost, oldest
secondary phloem become reactivated and form new cambium than produces new secondary xylem
and phloem. This type of secondary phloem located between two bands of xylem is called included
phloem.
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Unequal activity of the vascular cambium. In some woody vines, two sectors of the cambium are
very active, while two are almost completely inactive. The cambium grows outward in two
directions by remains thin in the other two, becoming flat and ribbon-like. This makes the stem
flexible and, at the same time, increases its conducting capacity as the elongating stem produces
more vascular tissue.
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Secondary growth in monocots. In some members of the monocot genus Yucca and Dracaena, a
type of vascular cambium arises in the cortex outside the vascular bundles. This cambium produces
parenchyma only. Columns of the parenchyma cells undergo rapid division and produce narrow
cells that differentiate into secondary vascular bundles containing xylem and phloem. The
outermost cells of each bundles develops fibers with thick secondary wall. The arrangement of the
secondary tissue is almost identical to that of the primary tissue.
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Unusual primary growth. The palm trunk is all primary tissue consisting of vascular bundles
distributed throughout ground tissue. A sheath of strong heavy fibers surrounds each vascular
bundle. Palm seedlings produce numerous adventitious roots from the base of the short stem. Each
root adds extra vascular bundles, and the portion of the stem above each new root can have that
many more bundles than it does below the root. This increase in width and addition of adventitious
roots in palms is called establishment growth, a form of primary growth.