Chapter 1 - Structure
Contents - Class Homepage
Sections:
Tree Meristems Leaves Shoots Roots Wood Bark

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.

Back to top 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).

Back to top 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.

Back to top 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.

Back to top 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...

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Back to top 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)

Back to top 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).

Back to top 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.