You should be able to do three things when you are done with this section:
1. Identify anatomical features of twigs, leaves, boles and bark and understand their function in tree survival and growth.
2. Identify patterns in development and growth of twigs, leaves, boles and bark.
3. Identify and know the consequences of tree growth abnormalities.
TREE
For our purposes, a tree is a woody plant that is 20+ feet tall at maturity,
with a single trunk that is unbranched for several feet, and with a more
or less well defined crown. Trees are the tallest and most massive plants
in the world. This is amazing, especially when you consider that during its
life a tree can not move (at least not very far). Trees not only survive
seasonal changes, but they may even have to cope with long-term climatic
changes!
GYMNOSPERMS
Tree species are split into two broad categories, based on differences in their
flowering and fruiting patterns. Gymnosperms are a taxonomic class that includes
plants whose seeds are not enclosed in an ovary. Gymnosperms (which translates
as "naked seeds") have an exposed ovule at the time of pollination.
Gymnosperms include pine, juniper, hemlock, spruce, cypress, fir and ginkgo
to name a few. Gymnosperm trees, with the exception of ginkgo, are also called
conifers because they bear their seeds in cones.
GYMNOSPERMS
Gymnosperms are often referred to as softwoods, because of their relatively
light weight but high strength which makes them very valuable for use in
the construction industry. You may have heard the term evergreen. The term
evergreen refers to the fact that these trees retain some of their leaves
for at least one winter. It surprises some people that evergreens do drop
needles in the fall, they simply do not drop all of them. Some gymnosperms
(e.g. larch, baldcypress) actually drop all their needles each fall. This
term evergreen includes most gymnosperms and some angiosperms growing in
warm climates.
On the left 2-year-old white pine needles turn yellow shortly before they fall off. The larch on the right has dropped all its leaves.
ANGIOSPERMS
Angiosperms are a taxonomic class of plants in which the ovule (which upon
fertilization becomes the mature seed) is contained inside an ovary. The
term hardwood is commonly used in reference to angiosperms, even though the
wood is often softer than softwoods. Some major hardwood genera include oak,
maple, hickory, birch, poplar, sweetgum and eucalyptus. "Angiosperm" translates
as "hidden seed".
ANGIOSPERMS
Trees in this group are also often referred to as deciduous. Deciduous trees
shed all their leaves during or before the onset of winter. However, some
angiosperms (e.g. rhododendron, southern magnolia) hold green leaves all
winter and recall some "evergreens" lose their leaves.
Rhododendron maintains green leaves all winter; however, when very cold the leaves hang almost lifeless but they never fall off.
ANGIOSPERMS
Dicots are a subdivision of angiosperms including plants (both tree and non-trees)
that have two cotyledons or seed leaves that emerge following seed germination.
Most trees are dicots. Plants in the other subdivision of angiosperms are
referred to as monocots. These include a few trees such as palm trees. The
older portions of palm tree trunks do not grow in diameter over the course
of their lives.
FORM
Trees are often divided into two classes based on their form. Excurrent trees
have a terminal leader that does not fork and grows consistently faster than
lateral branches, resulting in a crown of conical shape. Examples include
most conifers and some hardwoods, including yellow-poplar, sweetgum on the
left, and the pin oak on the right.
FORM
Trees with decurrent branching patterns have a spreading crown shape that is
the result of multiple forking of the terminal leader and growth of lateral
branches that is as fast or faster than the terminal leader such as this
American elm (left) or the sycamore (right).
MERISTEMS
People and all other animals grow in all parts simultaneously. Trees do not
grow like this. Trees produce new cells in a very limited number of places.
These places of cell division are called meristems. Meristems are zones of
rapid cell division and expansion. These zones are permanently juvenile and
undifferentiated. After meristematic division, expansion and differentiation,
cell maturation occurs. Cell maturation is usually what results in actual
plant growth.
MERISTEMS
The apical meristems are the most exciting area of cell division. Apical meristems
occur at the plant growing tips, both in the shoots and the roots. At the
apical meristems cells rapidly divide, and eventually give rise to new meristems.
Shoot meristems are located at the tips of growing twigs or encased in a
bud during periods of dormancy. Leaf meristems and branch meristems arise
when cells pull away from the apical dome. The protoderm and the procambium
also arise from the apical meristem. The protoderm becomes the cork cambium,
which actively divides to produce bark.
MERISTEMS
Directly inside the protoderm is the procambium, which becomes the vascular
cambium. Vascular cambium divides to produce phloem (outside) and xylem (inside).
Generally, apical meristems are considered primary meristems - they result
in upward or downward growth (primary growth). Cork and vascular cambium
are generally considered lateral meristems. Lateral meristems result in secondary
or diameter growth.
This board did not move up as the tree grew (height growth occurs at the tips), but it is being enveloped by diameter growth. On the right, the vascular meristem of this green ash has produced new wood which is slowly growing over the exposed wound.
MERISTEMS - BUDS
A bud is a small mass of meristematic tissue which develops in an embryonic
shoot and has the potential to produce primary growth. The entire structure
is wrapped in, or protected by, cataphylls. Cataphylls are a type of simple
leaf that subtends the tiny, preformed secondary leaves. Buds do provide
plants that produce them a distinct advantage. Buds allow plants to continue
to grow and produce tiny new leaves even when conditions are too nasty for
regular growth. Not all buds contain tiny leaves, some buds contain tiny
preformed flowers, or both leaves and flowers. Buds may be terminal (on the
end of the shoot) or lateral (on the side of the shoot, usually at the base
of the leaves).
MERISTEMS - BUDS
Buds may be dormant. Dormant buds begin as ordinary lateral buds that develop
in leaf axils but remain dormant and become overgrown. Dormant buds remain
alive and keep pace with tree diameter growth.
MERISTEMS - BUDS
When a tree with dormant buds is stressed, as is often the case with young,
suppressed, heavily thinned, or border trees, these dormant buds may grow
out. This outgrowth is referred to as either "water sprouts" or
epicormic sprouts. Epicormic sprouts result in degraded wood quality. Adventitious
buds are similar to dormant buds. They generally form on older portions of
the tree (either roots or stem), but unlike dormant buds they can not be
traced back to the pith. Adventitious buds may form root suckers, may form
stump sprouts when a tree is harvested, can form in response to wounding,
or may arise when roots are exposed to light. The growth of adventitious
buds is controlled by auxin, and their growth is often spurred by sudden
exposure to light.
Dormant buds have formed epicormic sprouts on the trunk of this pitch pine; on the right, adventitious buds have resulted in numerous sprouts on an apple tree.
MERISTEMS - APICAL DOMINANCE
A terminal leader is the uppermost, upwardly growing stem on a tree. In excurrent
trees, this branch is maintained in this position by exerting hormonal control
over the growth of subordinate branches. This hormonal control is often referred
to as apical dominance. Apical dominance is the phenomenon of the terminal
leader maintaining more rapid growth than lower branches as well as a more
upward orientation than laterals. Virginia pine is an example of a tree with
weak apical dominance.
LEAVES
Leaves develop from active meristems that are "sloughed off" the
apical dome, and consequently develop on the sides of apical meristems. These
sloughed off meristems form leaf primordia (embryonic leaves) that eventually
develop into leaves.
LEAVES
In angiosperms, the first leaves that are seen after a seed germinates are
the cotyledons. Because angiosperms are dicots, there are always two cotyledons.
As the tree grows, stipules and regular leaves are formed that are different
in morphology from the cotyledons.
This green ash seedling has simple juvenile leaves. It will later develop compound leaves.
LEAVES
Stipules are modified leaves that are found at the point of attachment of the
regular leaf. Stipules are rudimentary leaves that contribute carbon to spring
growth before leaves are fully expanded.
Sycamores have circular stipules.
LEAVES
In gymnosperms, the first leaves are also referred to as cotyledons. As the
seedling grows, the next type of leaf that is formed are the primary needles.
These needles are borne singly, even in species where later needles are born
in fascicles. Eventually, secondary needles may develop in the axils of primary
needles. In pines, these secondary needles appear as fascicles, or clusters
of needles, and are the type produced for the remainder of the life of the
tree.
Leaf Transection.
cuticle- waxy coating on the outside surface of leaves or bark Helps prevent desiccation.
epidermis- outermost layer of cells underlying the upper and lower leaf surfaces.
schlerenchyma- hardened cells provide structural support in leaves and other portions of the trees.
palisade cells- columnar cells in the leaves of angiosperms and some gymnosperms located beneath the epidermis. Palisade cells contain large amounts of chloroplasts.
xylem- a vascular tissue primarily responsible for carrying water and minerals.
spongy mesophyll cells- Chloroplast-containing, irregularly shaped cells adjacent to the lower epidermis.
guard cells- pairs of cells surrounding and regulating the size of stomatal apertures on leaf surfaces.
phloem- a vascular tissue primarily responsible for carrying carbohydrates.
stomata- pore on the leaf surface where most gas exchange takes place between interior leaf tissues and the environment.
resin ducts- Pockets of pitch (a sticky substance produced in conifers composed largely of oleoresins) which act to repel insects feeding on conifer needles.
transfusion tissue- a band of live and dead cells surrounding vascular tissue in pine needles.
endodermis - a ring of dermal tissue surrounding the vascular bundle.
chlorenchyma- chloroplast- containing cells in pines and some other gymnosperm leaves.
SHOOT
GROWTH PATTERNS
You may have noticed that there are several patterns of shoot growth. Several
species, especially those that grow in northern climates (including fir, spruce,
and eastern white pine) exhibit entirely fixed growth. In fixed growth, stem
units (stem + leaf primordia) are formed only after a bud forms in mid- to
late-summer. New leaves are formed under the bud until cold weather or drought
preclude their development. The new leaf primordia then overwinter in the bud,
and expand following budbreak in the spring. Because elongation occurs on a
year-by-year basis, the age of fixed growth species can often be determined
by counting branch whorls - they have one flush (whorl) per year. All of the
growth shown in the budbreak sequence for scarlet oak (below) is fixed growth.
Free growth may be observed in several species that grow in mild climates (including
red maple, poplar, apple and sweetgum). Free growth implies that stem units
are formed while elongation is taking place. These trees may also form a bud
in late summer and stack primordia that will elongate the following spring.
Trees may shift from free to fixed growth as they age. A good example of this
is spruce, which commonly exhibits free growth as a seedling, but switches
to solely fixed growth after several years, or when stressed.
SHOOT GROWTH PATTERNS
Somewhere between fixed and free growth species are the recurrently flushing
species, such as loblolly pine, longleaf pine, and northern red oak. Recurrently
flushing species may exhibit free growth during the growing season so long
as conditions are favorable. When conditions are bad, recurrent flushing
species may produce a temporary, or resting bud. While conditions are poor,
primordia are stacked under the resting bud. When conditions are once again
favorable, this resting bud flushes. A resting bud may become an overwintering
bud, failing to elongate in response to declining daylength. Recurrent flushers
may produce several flushes during the growing season, usually with the first
flush being the longest. Overall, these waves of growth produce elongation
for a longer period than purely fixed growth.
Recurrent flushing is common in shortleaf pine.
SHOOT GROWTH PATTERNS
Determinate growth is a pattern of bud development associated with fixed growth.
It implies the formation of a true terminal bud. This form of growth results
in relatively straight twigs, as with the maples, walnut, yellow-poplar and
willow. Indeterminate growth is a pattern of bud development associated with
free growth where twig elongation and bud formation continues until twig
growth is stopped by short days or frost. The portion of the twig beyond
the last lateral bud then dies. The last-formed lateral bud then acts as
a terminal bud when growth begins during the next growing season. Examples
include elms, birches and black locust. The end bud produced by indeterminate
species is referred to as a pseudoterminal bud.
This slippery elm twig still has its dead twig attached; the lateral bud shown will then be the pseudoterminal bud.
SHOOT GROWTH PATTERNS
Abnormal shoot growth may result from very late growing season outgrowth of
a bud that would have normally overwintered. This late season elongation
is commonly referred to as lammas growth. It may be caused by a variety of
factors, but is most often tied to late season rain and warm weather. It
is often seen in northern conifers that have been planted too far south.
This type of late season elongation is referred to as lammas growth when
it arises from terminal buds and prolepsis when it arises from lateral buds.
Late season elongation may result in winter damage or in stem forking. Proleptic
growth is premature elongation of a lateral winter bud. Proleptic growth
occurs in the late summer or fall and is usually caused by unusually large
amounts of rain late in the growing season.
SHOOT GROWTH PATTERNS
Foxtailing is the result of excessively strong apical dominance. It is commonly
observed in pines native to temperate zones that are planted in tropical
regions. Foxtailing results in long, unbranched sections of bole and poor
wood quality. Hormonal imbalances brought on by small variations in daylength
associated with the tropics are thought to be responsible.
Foxtailing in loblolly pine has resulted in 5 feet of unbranched growth.
Twig Parts:
terminal bud - The terminal bud is always at the apex of the twig. It contains the apical meristem, which is responsible for shoot growth. The morphology of the terminal bud is important for twig identification.
bundle scar - A bundle scar is a visible remnant of the connection between the leaf and stem vascular tissue on a twig. Bundle scars are completely enclosed by leaf scars.
stipule scar - The stipule scar is the wound remaining after the stipule falls from the twig. Not all twigs are stipulate. Stipules are rudimentary leaves that support growth in the spring before leaves are fully expanded.
lateral buds - Lateral buds are associated with leaves. They are involved with lateral branch and flower formation. Many lateral buds remain dormant; that is, they do not grow unless the terminal bud is damaged.
lenticels - Lenticels are pores in the twig surface. They allow oxygen and carbon dioxide to move in and out of succulent spring twigs. They are largely non-functional in older twigs.
leaf scar - The leaf scar is the wound that remains when the leaf falls from the tree in autumn. It is an important feature for tree identification, as its shape and appearance of vascular bundles varies by species.
bud scale scar - Not shown in this photo. A bud scale scar is a remnant of a terminal bud that forms a band around the twig. Terminal, pseudoterminal and resting buds can produce bud scale scars.
pith - Not shown in this photo. The pith is the central portion of a twig or root. Pith usually consists of tissue that is softer than surrounding growth or, in some cases such as honeysuckle, this tissue may be missing entirely.
stem unit- A collective term for one leaf (in angiosperms) or fascicle (in gymnosperms) and its associated portion of stem.
internode- the portion of a twig between two leaves in hardwoods or two fascicles
in gymnosperms.
Phyllotaxy (fill-oh-taxi) is a useful identification characteristic. Twigs can have either alternate or opposite phyllotaxy.
The following families have opposite phyllotaxy:
Aceraceae (the maples)
Oleaceae (the ashes)
Cornaceae (the dogwoods)
Caprifoliaceae (viburnums and honeysuckles)
Hippocastanaceae (horsechestnuts / buckeyes)
Scrophulariaceae (paulownia)
Bignoniaceae (catalpa - may be whorled)
Most other families are alternately arranged.
MAD Cap Horse is often used to remember the oppositely arranged trees.
LONG / SHORT SHOOTS
Spur shoots (short shoots) are produced by a pattern
of growth where lateral buds are restricted to small amount of elongation relative
to terminal buds. This differs from apical dominance in that short shoots
will not increase in growth rate if the terminal bud is removed. Examples
include apple, ginkgo and black gum. Long shoots are another term to describe
"normal" twig growth.
Here are some photos of spur shoots on apple and ginkgo...
. ROOTS
Roots have a lot of jobs. Roots serve as a means of anchorage,
they absorb water and minerals, they actively transport amino acids, water and
minerals, they store a variety of compounds, especially starch (think of a potato),
they are critically important sources of hormones and roots perceive drought.
All of this while working in an environment that is dark, dirty, wet, and with
little oxygen (like my office). However, the turnover rate is pretty
high. Most fine roots live for less than a year.
This is a taproot from a 6-inch diameter loblolly pine tree that was growing in heavy clay soil. We quit digging at four feet deep!
ROOTS
Root systems are composed of short-lived fine roots and longer-lived coarse
roots. Fine roots are defined as roots that are <1mm in diameter. They
are usually concentrated in the top 6 inches of soil. To give you an idea
of how numerous fine roots can be, a mature red oak may have 500 million
fine root tips! Coarse roots are far less numerous and may be found deeper
in the soil. Tap roots are one form of coarse roots that are produced by
some species in deep, well-drained soils. Tap roots grow nearly straight
down, providing support and allowing the tree access to deeper water. Roots
that grow from the tap root are referred to as lateral roots.
This is a taproot from a 6-inch diameter loblolly pine tree that was growing in heavy clay soil. We quit digging at four feet deep!
ROOTS
Roots of angiosperms typically grow in synchrony with the shoots. As with the
shoots, late summer droughts may slow root growth, and there is little growth
outside of the growing season. Gymnosperms produce new roots primarily in
the spring and fall. Generally, root growth is sensitive to temperature and
moisture fluctuations, with extremes of either slowing root growth.
ROOTS
As roots grow, the root apical meristem must push its way through the soil.
The growing tip is protected by the root cap and pushes its way forward by
the expanding cells behind the apical meristem.
ROOTS
Measuring and observing roots
is a difficult and painstaking task. Rhizotrons are often used to to
collect root growth measurements over time. A rhizotron (Greek, from rhiza,
root)
is an enclosure with at least one transparent panel that allows for non-invasive
viewing of underground processes, specifically root systems.
Shown above is a "minirhizotron" system. Access tubes are installed in the ground and a small camera captures pictures along the length of the tube. The images are stored on a laptop computer. To the left is an image of green ash roots clearly showing a secondary root with several primary roots with apical meristems.
ROOTS
A root's ability to take up water changes as the root ages. Generally, very
young root tissues take up water readily as a result of a water potential
gradient (see the section on water). As the root tissue ages, the cells become
suberized. That is, a wax-like layer develops that allows the roots to survive
the winter and extended droughts. In suberized roots, a structure referred
to as a casparian strip develops. The casparian strip is a zone in which
the intracellular spaces are sealed by a band of suberized material around
the cell walls. Very little water moves through the suberized roots. In older
roots, cracks and lenticels provide places for water entry into the root.
In these zones, water is pulled into the root from the soil by decreased
pressure in the xylem (see the section on water). Very young roots have root
hairs, which result in tremendous surface areas and water uptake.
ROOTS
Mycorrhizal associations further increase the surface area in these young roots.
Mycorrhizae are fungi that have a symbiotic relationship with tree roots.
Trees supply the fungus with carbohydrate, and the fungus supplies the tree
with increased water and nutrient absorption, increased resistance to sulfur
and aluminum toxicity, and increased tolerance of soil pH. There are two
classes of mycorhizal fungi. Ectotrophic fungi are visible to the naked eye.
This type of fungi surround the root to form a fungal sheath, and extend
fungal hyphae into the soil and into intracellular spaces (forming a "Hartig
net").
Ectotrophic fungi on pine roots.
ROOTS
Ectotrophic mycorrhizae are commonly seen on pines, fir, spruce, birch, oak
and hickory.
The other type is the vesicular - arbuscular (VA or endotrophic) fungi. VA fungi are so named because they form both vesicles (ovoid structures) or arbuscles (branched structures) inside cells. This type of mycorrhizae do not grow exclusively inside cells, but also extend hyphae out into the soil. This type of fungus is commonly seen in maple, sweetgum and yellow-poplar.
Ectotrophic fungi on pine roots.
ROOTS
Another type of root symbiotic association occurs between several species of
trees and nitrogen fixing bacteria. In these associations, nodules are produced
by the roots of the host plant upon bacterial infection. Bacterial cells
are actually incorporated into the cytoplasm of the host cells. From this
association, the host plant may gain usable ammonia from unusable atmospheric
nitrogen through a process called nitrogen fixation (N2 -> NO3 and NH4).
The bacteria gain a home and a steady source of food from the host plant. Excellent
examples of this type of association occur between black locust (Fabaceae family)
and the bacterial genus Rhizobium and between the alders (Alnus) and the actinomycete
Frankia. The association between Fabaceae and Rhizobium may produce 60 - 100
kg/ha/yr of usable nitrogen! This association may be important on very nitrogen
- poor sites such as mine spoils and impoverished sandy soils.
ROOTS
These are root nodules on European black alder seedlings. Root nodules are
actually modified roots and continue to grow and divide each year. The nodules
contain an actinomycete microorganism. The microorganism exists symbiotically,
using fixed carbon from the plant and producing usable nitrogen.
ROOTS
Buttressed roots are swellings at the base of some shallow-rooted species that
increase the tree's ability to withstand high winds and aid in the aeration
of submerged root systems. Excellent examples include water tupelo and baldcypress.
Baldcypress also develops cypress "knees" which result from rapid
growth of root meristems on the top surface of large lateral roots. These "knees" are
also thought to aid in aeration and windfirmness.
ROOTS
Several species may produce root grafts. That is the root systems of two individuals
that come into contact may grow together, becoming in effect one organism.
This is common in Douglas-fir, and cut stumps adjacent to live trees may
actually grow over and heal, much like a wound! Grafted roots can also be
responsible for spreading vascular diseases from one tree to another. This
often occurs in Dutch Elm disease.
ROOTS
Aerial roots are roots that are exposed above the soil surface due to erosion
of soil mineral or organic matter where rooting had occurred. These are often
observed in yellow birch, red spruce (below) and other species that seed
in on nurse logs or that grow on thin organic soils. However, some groups
(such as Ficus) may produce roots along the bole and branches without any
soil at all!
Aerial roots on strangler fig (Ficus aurea)
WOOD
COMPONENTS
Time to look at the tree bole. The bole is the main stem of a sapling or mature
tree (a bole is forestry jargon for a tree trunk). You may recall that xylem
(which transports water and minerals) is formed to the inside of the vascular
cambium and phloem (which transports food) is formed to the outside of the
vascular cambium. Generally, there are 10-15 times more xylem cells formed
than phloem cells, but this ratio can fall to 2:1 under stress. In the gymnosperm
group, only one type of cells is formed. Tracheids serve both as conductive
and support cells. In the angiosperms, vessels and fibers are formed in addition
to tracheids. Vessels are very wide and short specialty cells that are adapted
to move large amounts of water. Fibers are narrow and tough cells that function
as support.
Tracheids are long and narrow and not as efficient and conducting water as shorter and much wider vessel elements (far right) found in angiosperms.
Schematic of a coniferous tree with a fixed growth pattern. One flush is produced each year. Note the manner in which the trunk increases in thickness (not to scale) and in height.
WOOD COMPONENTS
Depending on the distribution of the wide vessels, wood may be classified as
diffuse porous or ring porous. Diffuse porous species produce vessels slowly,
with even and uniform vessel production throughout the season. Examples include
poplar, maple and birch. Ring porous species produce large vessels in the
spring and smaller diameter vessels later. Examples include oak, ash and
elm. Another obvious feature in wood is ray parenchyma. It is comprised of
radially-oriented plates of living cells in secondary xylem. Ray parenchyma
stores and transports carbohydrates and other compounds.
Stem transection showing variation in vessel diameters and distribution within annual growth increments of a diffuse-porous species, silver maple (left) and a ring-porous species, white oak (right) (x50). Photo courtesy of U.S. Forest Service.
WOOD COMPONENTS
At some time during your life you must have seen annual (growth) rings, and
you may know that each growth ring represents a year. What makes these rings?
An annual ring is a year worth of secondary xylem growth. Each year of growth
can be easily spotted by looking for the spring transition from latewood
to earlywood as the tree starts growing. Earlywood is formed when the tree
is rapidly growing in diameter in the springtime.
WOOD COMPONENTS
Earlywood cells are large, of low density and have thin cell walls. Latewood
is formed later in the summer. Latewood cells are smaller, high density cells
that have very thick cell walls. The transition from earlywood to latewood
may be quite sharp. During some years, early drought may slow tree growth
and produce latewood early in the year. If a period of wet weather follows
this, a false ring may be produced.
In this weathered post the latewood is quite obvious. Smaller, high-density, thick-walled latewood cells are much more resistant to weathering than the thinner-walled earlywood cells.
WOOD - DENDROCHRONOLOGY
By studying a trees growth rings, or by studying the growth rings of trees
that have long-ago died, we can determine when events occurred that influenced
tree growth. This is called dendrochronology. With
dendrochronology we can determine when a trees neighbors were removed (a jump
in tree ring width), which years were droughty, and what years were wet.
Dendrochronology has been used to date hurricanes in wind resistant bald cypress
in the Mississippi delta. How?
Surrounding trees such as red maple and tupelo blow down. This "thins" the
stand and results in faster growth in the remaining cypress trees, which is
evident in wider growth rings.
Dendrochronologists are examining tree ring patterns using this microscope and television monitor which projects the image
WOOD - REACTION WOOD
Snow and ice may often bend stems, resulting in abnormal secondary xylem production.
Reaction wood may be formed in response to this bending and, if the damage
is less severe, may eventually result in straightening the stem.
In hardwoods, tension wood is formed on the upper side of leaning stems. In conifers, compression wood is formed on the lower surface of leaning stems.
WOOD - REACTION WOOD
In cross section, reaction wood looks and behaves differently from ordinary
wood. In softwoods, wider growth rings may be observed on the lower side
of the stem, and there is a greater percentage of latewood. In hardwoods,
growth rings are wider on top, and boards cut from logs with tension wood
may have "fuzzy" surfaces and decreased strength.
WOOD - HEARTWOOD
In looking at a tree cross section, you may notice that the wood nearest the
bark is light in color, and the wood closer to the center of the tree is
darker. The lighter, younger wood is referred to as sapwood. Sapwood is composed
of active xylem, live storage cells, and ray parenchyma (used for storage
and the movement of chemical compounds toward the heartwood). In sapwood
roughly 10% of the cells are still alive. The darker wood is referred to
as heartwood. This wood is made up of old secondary xylem in which all of
the cells have died. This region plays no role in conducting water or minerals.
Heartwood often contains high concentrations of oils, gums, tannins and resins
which are toxic to decay fungi and give the wood a dark color.
The heartwood in black walnut is very dark and distinct, making the wood very valuable.
The sapwood in this apple limb is clearly visible since it became wet and darker minutes after the cut was made.
CELL
WALLS
Cell wall organization.
Idealized model of typical wall structure of a fiber or a tracheid. The
cell wall consists of: P (primary wall), S1, S2, and S3 (layers of the secondary
wall), W (warty layer - not always evident), ML (middle lamella - the high-lignin
material that bonds cells together).
BARK
STRUCTURE
Cork cambium is located outside of the vascular cambium. Cork cambium produces
new cells to the outside that eventually become bark. Bark actually has a lot
of the same functions as your skin. Bark prevents water loss, it protects the
tree against fungal decay, protects against insect mechanical injury, and (if
it is thick enough) can act as insulation against fire. Lenticels are a structure
that you often see in relation to the bark. Lenticels are actually pores in
the twig surface. They allow oxygen and carbon dioxide to move in and out of
succulent spring twigs. They are largely non-functional in older twigs. Some
young twigs, and even older twigs in some species, may be green. Some green
twigs actually carry on photosynthesis!
BARK STRUCTURE
Bark can be thought of as three distinct layers. The phellogen (cork cambium)
produces cork cells. The phelloderm is the layer just to the inside of the
phellogen. The phelloderm is also alive, and may carry on photosynthesis
as well as store starch. The outermost layer is the phellem. The phellem
is the dead layer on the outside of the tree. These three layers together
make up the periderm, which replaces the epidermis. The bark that you see
consists of alternating layers of periderm and dead phloem.
BARK STRUCTURE
Bark characteristics are dictated by the pattern of periderm formation. More
continuous periderms result in bark that peels in large sheets (Betula, Lonicera,
Vitis).
An added advantage for trees that shed their bark is the removal of waste products.
MORPHOLOGY
Tree bark can vary greatly throughout the life of a tree,
often becoming rougher as a tree grows. Here are examples of just a few of the
many different bark types.
Beech bark stays smooth its entire life, as can be seen on this older tree.
Shagbark hickory shreds its bark in long strips as it gets older.
Pitch pine has large plates. Epicormic sprouting is also common as can be seen on the left side of this tree.
Tulip-poplar bark displays a pattern of interlacing ridges and furrows that form a diamond shaped pattern.
Persimmon has small blocky plates and often looks like charcoal briquettes.
Yellow birch has a very fine shreddy bark that peels into small strips.