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STEMS
Chapter 6
FUNCTION OF THE STEM
1. Support of the photosynthetic, reproductive and storage parts.
2. Conduction of water and metabolites.
3. Production of new stem tissues.
EXTERNAL MORPHOLOGY (see fig. 6.1)
1. Terminal and axillary (lateral) buds.
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Covered with bud scales while dormant.
Contain meristem and produce primary tissues.
Lateral buds are associated with leaf axils.
Bud scale scars.
2. Nodes and internodes.
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Nodes are the regions of leaf attachment.
Internodes are the space between two nodes.
3. Leaf scars and bundle scars.
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Bundle scars mark the location where the vascular tissue passed to the leaf.
There may be one or many bundle scars.
The number and arrangement of the bundle scars is characteristic of the
species.
4. Lenticels.
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Loosely arranged cells that allow gas exchange.
Broken epidermis.
Leaves usually have a flattened blade, and are attached to the twig by a stalk called the
petiole
The angle between the petiole and the stem contains a bud, a lateral or axillary bud.
The angle between the petiole and the twig is called the axil.
The apical bud, when present, is partially responsible for inhibiting the growth of the lateral
buds. This is called apical dominance.
Most buds are protected by bud scales, which fall off when the bud starts to grow.
The apical bud makes the twig grow longer; axillary buds develop into branches.
The base of the petiole may have stipules, small appendages that are sometimes leaf-like.
Stipules may fall off after a while or remain through the live of the leaf. They usually leave
tiny scars around the twig.
Some stems are modified to perform other functions.
ORIGIN AND DEVELOPMENT OF STEMS
The apical meristem is located in the apical bud and is protected by the bud scales.
The buds also contain leaf primordia (singular primordium) that develop into leaves and
bud primordia that develop into lateral shoots. See fig.6.2
The apical meristem undergoes mitosis and produces three primary meristems:
1. The protoderm that gives rise to the epidermis. The epidermis is usually made of a
single layer of cells and becomes covered with the cuticle.
2. The procambium is a cylinder of strands inner to the protoderm that gives rise to the
vascular tissue. Primary xylem and primary phloem are produced by the procambium.
3. The ground meristem produces two tissues: the pith in the center of the stem,
surrounded by the vascular tissue; and the cortex between the vascular tissue and the
epidermis.
Pith cells may breakdown shortly after being formed leaving a hollow center.
The cortex may become extensive in herbaceous plants, but in woody dicots it eventually
disappears and is replaced by secondary growth.
Pith and cortex usually function as storage tissue.
Five primary tissues are produced in all by the apical meristem: epidermis, primary xylem,
primary phloem, cortex and pith.
These five tissues are produced from the primordia through the process of differentiation.
During differentiation, a strand of xylem and phloem called a trace, grows from the central
vascular bundle and enter the leaf or bud.
Each trace leaves a little gap in the vascular cylinder. These gaps are filled with
parenchyma tissue and are called leaf or bud gaps.
The elongation of the stem occurs by internodal elongation, which may occur
simultaneously over several internodes.
Increase in stem thickness is the result of cell division and cell enlargement.
In plants with secondary growth this primary thickening is moderate.
In monocots, like palms, there is a massive primary growth.
A narrow band of cells between the primary xylem and the primary phloem may retain its
meristematic nature and become the vascular cambium, one of the two lateral meristems.
The tissue produced by the vascular cambium is secondary tissue and increases the girth
of the stem.
Vascular cambium produces xylem to wards the inside of the stem and phloem towards the
outside of the stem. The cambium layer separates the xylem from the phloem.
In most woody stems and root, periderm formation follows the beginning of secondary
growth in the xylem and phloem.
The periderm consists of three layers:
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Cork cambium of phellogen.
Cork or phellem.
Cork parenchyma or phelloderm.
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.
Lenticels allow gas exchange through the periderm.
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.
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 if 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 dicots survive underground during the unfavorable season.
Woody dicots survive above ground but usually stop growing during the unfavorable
season.
Woody perennials flower only when they are adults, which may take many years.
TISSUE PATTERNS IN STEMS
STELES
Primary xylem, primary phloem and pith, if present, make up a central cylinder called the
stele.
Protostele consists of solid core of conducting tissues in which the phloem usually
surrounds the xylem.
Siphonosteles have a pith in the center.
Differences between dicots and monocots.
MONOCOTS
Flower parts usually in multiples of three.
five.
Pollen with single furrow or pore.
Embryo with single cotyledon.
Major leaf veins parallel
Stem vascular bundles scattered
Roots are adventitious, fibrous system.
Secondary growth absent
DICOTS
Flower parts usually in multiples of four or
Pollen with three furrows or pores.
Embryo with two cotyledons.
Major leaf veins reticulated
Stem vascular bundles in a ring
Roots, taproot system.
Secondary growth commonly present
Herbaceous dicotyledonous stems
Herbaceous dicots form a ring of discrete vascular bundles around the pith.
Most of the tissue of herbaceous dicots is primary but the cambium may produce some
secondary growth.
The interfascicular parenchyma is wide separating the vascular tissue into discrete
bundles around the pith. The wood is soft.
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The cambium layer is located between the primary xylem and phloem; in some
species, the cambium layer extends between the bundles forming a ring.
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Xylem differentiate close to the pith and phloem differentiates next to the cortex.
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The stems of some herbaceous dicots have little or no secondary growth.
Woody dicotyledonous stems
Wood is secondary xylem.
Secondary growth is an increase in stem and root girth.
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It occurs in woody plants only.
It is due to the activity of the lateral meristems: vascular cambium and cork
cambium.
Vascular cambium forms a cylinder along the length of roots and stems, between
the xylem and phloem; it produces more xylem and phloem.
Cork cambium is located in the outer bark.
Vascular cambium is located between the xylem and the phloem.
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Secondary xylem (wood) is produced to the inside.
Secondary phloem (inner bark) to the outside.
Ray initials are cambium cells that produce rays of parenchyma cells.
Fusiform initials produce new xylem and phloem.
Rays are chains of parenchyma cells that radiate out of the center of the stem and
transport materials laterally between the secondary xylem and secondary phloem
1. 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.
Differential growth of spring or early cells and autumn or late cells forms the rings.
Some angiosperms produce larger vessels early in the season (spring wood) than
those late in the season (summer wood). These trees produce ring-porous wood.
Other angiosperms produce vessels of uniform size and distribution. These are called
diffuse-porous wood.
2. Young and old wood.
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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 into the cavity
of a vessel.
Bark refers to all tissues external to the vascular cambium: secondary phloem, cork
cambium and cork (phloem plus periderm).
The bark includes all tissues outside the vascular cambium that is phloem, cortex and
periderm.
Each year the vascular cambium adds secondary phloem to the outside area.
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.
Cork cambium is located near the surface of the stem.
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Cork cells are produced toward the outside.
Cork cambium and cork make the periderm.
The first cork cambium may remain active for many years (20 years) as in apples and
pears.
In other trees, new cork cambium is formed deeper in the bark from phloem parenchyma
cell that become meristematic and are no longer active in transporting materials in the
stem.
The cork cambium separates the living inner bark from the dead outer bark.
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
phloem.
Phloem 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 phloem.
Laticifers are specialized cells or ducts found in about 20 families of plants.
Laticifers are commonly found in the phloem but they may occur anywhere in the plant.
Laticifers produce latex, a complex mixture of gums, proteins, sugars, oils, salts, alkaloids
(secondary metabolites), enzymes and other substances.
Latex may be white, yellow, orange or red.
See additional information about latex:
http://www.smtl.co.uk/MDRC/Gloves/jowcpaper96/jowc-glove-paper-what-is-latex.html
http://www.life.uiuc.edu/ib/363/rubber.html
http://www.life.uiuc.edu/ib/363/RESINS.html
Monocotyledonous stems
Most monocots are herbaceous plants that do not attain great size.
Their stems lack both vascular and cork cambium, and, therefore, produce no secondary
growth.
The vascular bundles are scattered throughout the ground tissue.
The bundle contains two large xylem vessels and is surrounded by a sclerenchyma sheath.
The xylem is always closer to the center of the stem, and the phloem closer to the surface.
The parenchyma tissue between the bundles is not separated into cortex and pith.
Secondary growth in some monocots
In monocots, like palms (Arecaceae) and screw pines (Pandanaceae), there is a massive
primary growth.
“Unlike most monocots, palm stems can grow in girth by an increase in the number of parenchyma cells and
vascular bundles. This primary growth is due to a region of actively dividing meristematic cells called the
"primary thickening meristem" that surrounds the apical meristem at the tip of a stem. In woody monocots this
meristematic region extends down the periphery of the stem where it is called the "secondary thickening
meristem." New vascular bundles and parenchyma tissue are added as the stem grows in diameter.”
http://waynesword.palomar.edu/trjune99.htm
There is secondary growth in all the genera of the Agavaceae and some of the
Xanthorrhoeaceae, Liliaceae and Iridaceae.
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Cordyline (Agavaceae) produces secondary vascular tissue.
Monocot stem xs with secondary vascular bundles x40. This is a palm-like plant
(Beaucarnia) in the family Agavaceae. The entire (but mainly the outer) cortex remains
meristematic and produces secondary vascular bundles, resulting in an increase in stem
diameter. In Monocots there is no single vascular cambium like that found in virtually all
other plant groups with secondary growth.
Monocot secondary vascular bundles x100. These Beaucarnia secondary vascular bundles
have a small patch of phloem in the center that is completely surrounded by xylem
MODIFIED STEMS
Rhizomes are elongated horizontal underground stems bearing buds in the axils of
reduced scale leaves, e.g. iris.
Stolons and runners are slender stems growing along the surface of the ground that root
and produce a plantlet, e.g. strawberry, Boston fern.
Tubers are swollen ends of underground stems used in storage and reproduction, e.g.
potato.
Bulbs are small conical stems with numerous storage leaves attached to it, e.g. onion,
garlic.
Corms are thickened rounded storage stems covered with thin papery scales, e.g. crocus,
gladiolus
Cladophylls are flattened stems that appear leaf-like, e.g. the “leaves” of asparagus are
cladophylls, modified stems.
Thorns are modified stems, e.g. honey locust.
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Thorns are different from spines which are modified leaves like in cacti, and
prickles which are epidermal outgrowths, like in roses.
Some tendrils are modified stems, e.g. grape vine, but in other plants are modified leaves.
Tendrils are used for support of the vine.
WOOD AND ITS USES
Density is the weight per unit of volume.
Specific gravity is the ratio of the weight of a substance to the ratio of an equal volume of
water.
Oven-dry weight of wood
Specific gravity = ------------------------------------Weight of the displaced volume of water.
The difference in the specific gravity of wood depends on the proportion of wall substance
to lumen.
Presence of fibers increases the specific gravity.
Presence of large vessels lowers the specific gravity.
Woods with specific gravity of less than 0.50 are considered light; those with gravity greater
than 0.70 are considered heavier.
The wood’s ability to withstand decay caused by organisms and insects is referred to as its
durability.
Tannins and oils repel insects.
Logs are usually cut longitudinally (quatersawed) along the radius or perpendicular to the
rays (tangentially).
Knots are the bases of old, fallen branches that have been covered with new wood.
Gross features of wood.
Color is variable and depends of cell wall deposits as well as deposits in the heartwood.
Grain refers to the alignment of the axial cells: vessels, tracheids, etc.
Texture depends of the size of the elements.
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Fine texture occurs when the vessels are small and the rays narrow.
Coarse texture occurs when the vessels form wide bands and the rays are broad.
Uses of wood:
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About half the timber produced in the United States and Canada is used as
lumber.
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Sawdust and waste are converted to particle board and pulp for paper,
synthetics, and linoleum.
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Other timber is used for charcoal, railroad ties, boxes, tool handles, and so
forth.
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Developing countries use a greater proportion of their timber for fuel.