Wood Structure

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The Anatomy Of Wood

Microscopic Structure & Grain Of Wood

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The microscopic cellular structure of wood, including annual rings and rays, produces the characteristic grain patterns in different species of trees. The grain pattern is also determined by the plane in which the logs are cut at the saw mill. In transverse or cross sections, the annual rings appear like concentric bands, with rays extending outward like the spokes of a wheel.

Cross (transverse) section of a grand fir (Abies grandis) log in the Pacific northwest forest of North America. The annual rings appear like concentric bands and can be counted to age-date the tree. The darker wood is called heartwood, while the lighter wood is called sapwood.

This basswood (Tilia americana) trunk cross section has 24 distinct annual rings. The central core of wood (#1 in close-up photo) counts as the first year of growth since the pith is no longer present. The smaller series of concentric rings (knot) at the bottom of the photo is a lateral branch embedded in the main trunk.

Cross (transverse) section of California coast live oak (Quercus agrifolia). The annual rings appear like concentric bands and can be counted to age-date the tree. This is a ring-porous wood, with bands of large, porous spring vessels. Smaller, dense tracheids and vessels occupy the wider gaps between the spring bands. In this wood, the spring vessels actually appear darker and are easier to count. In pine wood, the darker, summer bands are easier to count.

This small block of angiosperm wood is used for an aquarium aerator. Fine jets of air bubbles come out of the porous vessels from the transverse surface of the block.

Lack Of Visible Annual Rings In Tropical Trees?

In the tropical rain forest, relatively few species of trees, such as teak, have visible annual rings. The difference between wet and dry seasons for most trees is too subtle to make noticeable differences in the cell size and density between wet and dry seasonal growth. According to Pascale Poussart, geochemist at Princeton University, tropical hardwoods have "invisible rings." She and her colleagues studied the apparently ringless tree (Miliusa velutina) of Thailand. Their team used X-ray beams at the Brookhaven National Synchrotron Light Source to look at calcium taken up by cells during the growing season. There is clearly a difference between the calcium content of wood during the wet and dry seasons that compares favorably with carbon isotope measurements. The calcium record can be determined in one afternoon at the synchrotron lab compared with four months in an isotope lab.

Poussart, P.M., Myneni, S.C.B., Lanzirotti, A., et al. 2006. Geophysical Research Letters 3: L17711.

In a tree trunk, all the tissue inside the cambium layer to the center of the tree is xylem or wood. All the tissue outside the cambium layer (including the phloem and cork layers) is the bark. Some botanists prefer to use the term phellem for the corky bark layer because it develops from a special meristematic layer outside the phloem called the phellogen. The wood of a tree trunk is mostly dead xylem tissue. The darker, central region is called heartwood. The cells is this region no longer conduct water. They appear darker because they often contain resins, gums and tannins. The lighter, younger region of wood closer to the cambium is called sapwood. Although they are dead, the cells in this region serve as minute pipelines to conduct water and minerals from the soil. Xylem cells are alive when they are initially produced by the meristematic cambium, but when they actually become functioning water-conducting cells (tracheids and vessels), they lose their cell contents and become hollow, microscopic tubes with lignified walls. The structure of plant stems is explained in more detail in the following article.

The Rise of Water In Plant Stems

Water is often excreted through special pores in the leaves and stems called hydathodes as a result of root pressure within the xylem tissue. This process is called guttation, and it occurs in many species of plants. When soil moisture is high and transpiration is low, water enters the roots and can be forced out the ends of veins in leaves to produce the water droplets. This may also occur at night when transpiration is normally shut down. The classic example of guttation is droplets at the tip of grass leaves in the morning. This is not water condensation (dew) from the air.

Guttation at the base of orchid flower stalks (pedicels).

Root pressure does not adequately explain the rise of water in plant stems. In fact, the pressure required to force water up tall stems would greatly exceed the force of root pressure. In addition, root pressure does not operate when soil moisture is low, and even when soil moisture is high it is too weak to force water up a tall plant. Water molecules are actually pulled up from the leaves through minute tubular cells of the xylem tissue.

The rise of water in plant stems is a function of the polarity of water molecules and the small bore diameter of tracheids and vessels in xylem tissue. Water molecules have a positive and negative end, and literally stick together (cohere) like molecular magnets. When water is confined to tubes of very small bore, the force of cohesion between water molecules is very strong. Tensions as great as 3,000 pounds per square inch are needed to break the column of water molecules. This is roughly equivalent to the force needed to break steel wire of the same diameter. In a sense, the cohesion of water molecules gives them the physical properties of solid wires.

The rise of water in plant stems cannot be compared with a vacuum pump because the maximum height for a vacuum pump is only 34 feet. Water transport in nonvascular plants without tracheids and vessels is accomplished primarily by osmosis and imbibition, where water simply soaks up into the plant tissue like a sponge. This explains the ascent of water in mosses and liverworts (phylum Bryophyta), but does not account for the rise of water in tall trees and shrubs. The following explanation for the ascent of water in plants is summarized from the Transpiration Pull-Cohesion Theory, also known as the Cohesion-Tension Theory:

When water evaporates from the mesophyll cells of a leaf and diffuse out of the stomata (transpiration), the cells involved develop a lower water potential than the adjacent cells. Because the adjacent cells then have a correspondingly higher water potential, replacement water moves into the first cells by osmosis. This continues across rows of mesophyll cells until a small vein is reached. Each small vein is connected to a larger vein, and the larger veins are connected to the main xylem in the stem, which in turn is connected to the xylem in the roots that receive water, via osmosis, from the soil. As transpiration takes place it creates a "pull" or tension on water columns, drawing water from one molecule to another all the way through the entire span of xylem cells. The cohesion required to move water to the top of a 300 foot redwood tree is considerable.

Water is primarily "pulled" upward due to the cohesion of water molecules within the xylem tracheids and vessels. Like a steel wire, the chain of water molecules is literally pulled through the plant's vascular system, from the roots to the leaves. As water molecules move out through the stomata into the atmosphere, they are replaced by new molecules entering the roots from the soil. Since the water in xylem ducts is under tension, there is a measurable inward pull (due to adhesion) on the walls of the ducts. It has been estimated that only about one percent of all water molecules transported upward are used by a tree; the other 99 percent are needed to get that one percent up there. Water molecules must literally grow with the plant in order to form continous chains within the xylem tubes.

According to George Koch of Northern Arizona University and his associates, there may be a limit to the maximum height of tall trees. [Koch, G.W., Sillett, S.C., Jennings, G.M. & Davis, S.D., 2004. "The Limits to Tree Height." Nature 428: 851-854.] They climbed to the top of the tallest redwoods and measured the water potential and photosynthesis in the highest branches. They concluded that gravity starts to win out against water cohesion at about 110 meters (360 feet). This value correlates with the fossil record for tall trees at about 120 meters. The hydrogen bonds between water molecules become insufficiently strong to hold the cohesive mass of water molecules below the leaves. In addition, a decrease in the water potential of leaf cells causes the stomata to close, thus restricting water loss and the availability of carbon dioxide. The reduction of carbon dioxide shuts down photosynthesis, and this may also limit the growth and height of a tree.

The tallest living California coast redwood (Sequoia sempervirens) on record stands 379 feet (116 m), 64 feet (20 m) taller than the Statue of Liberty. California redwoods are rivaled in size by the amazing flowering Australian tree (Eucalyptus regnans). The record for the tallest tree of all time has been debated by botanists for centuries. Some amazing claims for towering Douglas fir (Pseudotsuga menziesii) and E. regnans exceeding 400 feet (122 m) have never been substantiated by a qualified surveyor. In 1872, a fallen E. regnans 18 feet (5.5 m) in diameter and 435 feet (132 m) tall was reported by William Ferguson, making it the tallest (or perhaps longest) dead tree. According to the monograph on Eucalyptus by Stan Kelly (Volume 1 of Eucalypts, 1977), trees of E. regnans well over 300 feet (91 m) tall have been measured, but the tallest tree known to be standing at present is 322 feet (98 m). Since E. regnans is a flowering plant (angiosperm), it has vessels and tracheids, while gymnosperms such as redwoods have tracheids but no vessels. It is interesting to speculate about which of these two trees has the tallest growth potential.

Giant coast redwoods (Sequoia sempervirens) at Henry Cowell Redwoods State Park and Big Basin Redwoods State Park in Santa Cruz County, California. The largest trees are over 300 feet (91 m) tall and 17 feet (5.2 m) in diameter. They sprouted from seeds the size of oatmeal flakes nearly 2000 years ago, and grew into giants taller than the Statue of Liberty (from the foundation of pedestal to torch).

A recent article in Science Vol. 291 (26 January 2001) by N.M. Holbrook, M. Zwieniecki and P. Melcher suggests that xylem cells may be more than inert tubes. They appear to be a very sophisticated system for regulating and conducting water to specific areas of the plant that need water the most. This preferential water conduction involves the direction and redirection of water molecules through openings (pores) in adjacent cell walls called pits. The pits are lined with a pit membrane composed of cellulose and pectins. According to the researchers, this control of water movement may involve pectin hydrogels which serve to glue adjacent cell walls together. One of the properties of polysaccharide hydrogels is to swell or shrink due to imbibition. "When pectins swell, pores in the membranes are squeezed, slowing water flow to a trickle. But when pectins shrink, the pores can open wide, and water flushes across the xylem membrane toward thirsty leaves above." This remarkable control of water movement may allow the plant respond to drought conditions.

Syrup From The Sugar Maple (Acer saccharum)

In sugar maple (Acer saccharum), a deciduous tree of the midwestern and eastern United States, cells of the sapwood at the base of the tree produce large amounts of sugar during late winter and early spring. The sugary sap results from the conversion of starches accumulated during the previous growing season into sugars during the winter, mostly in ray cells. During March and April, when the ground is thawing and the sap is flowing, holes are drilled into the sapwood at the base of the trunk. A tube or spigot (called a spile) is inserted into the hole and a pail hung below it. The watery sap drips down the spile and into the bucket. The sap is boiled down until it reaches the desired consistency for maple syrup. Most commercial syrups are sweetened and thickened with corn syrup and water soluble gums (such as cellulose gum). They are usually colored and flavored with caramel color and natural or artificial maple flavoring. They often appear darker and thicker than pure maple syrup. Maple sugar comes from the evaporated maple syrup.

A decorative bottle of golden maple syrup and a sugar maple tree (Acer saccharum) during the fall in Indiana. Maple syrup is tapped from the sapwood during early spring (March-April) when the ground is thawing and the sap is flowing.

Transverse section from the stem of a Central American logwood tree (Haematoxylum campechianum). The dark, reddish-brown heartwood contains a valuable dye that was a major factor in the colonization of British Honduras and the subsequent establishment of the nation of Belize.

Wood is cut longitudinally in two different planes: tangential and radial. Tangential sections are made perpendicular to the rays and tangential to the annual rings and face of the log. This plane is also called slab-cut or plane-sawed lumber. The annual rings appear in irregular, wavy patterns. This is the plane in which most lumber is cut at the saw mill. In the manufacture of plywood, thin sheets of veneer are peeled off of a rotating tree trunk. The sheets of veneer are glued together with the grains of each sheet at right angles to each other. An odd number of sheets produces 3-ply and 5-ply boards. The alternation of sheets greatly increases the strength and durability of plywood lumber. With modern glues, particles and chips of wood are also cemented together to form strong particleboards. Different grades of particleboard contain different sized wood chips. Fiberboard differs from particleboard in that wood fibers, not chips of wood, are used.

A sheet of coarse particleboard made from wood chips glued together. The use of wood chips provides a cost effective method of increasing the lumber yield from forests.

Tangential grain pattern of Douglas fir (Pseudotsuga menziesii) showing the attractive, wavy pattern of annual rings on the outer veneer layer of the plywood. The board consists of five layers of veneer glued together at right angles to each other.

Petrified sequoia wood (Sequoia) showing perfectly preserved tangential (T) and radial (R) planes. This 15 million-year-old petrified wood was uncovered from its ancient tomb of flood sediments and lava flows near the Columbia River Gorge in central Washington. Sequoia trees once grew wild in this region 150,000 centuries ago.

Radial sections are made along the rays or radius of the log, at right angles to the annual rings. This plane is also called quarter-sawed lumber because the logs are actually cut into quarters. The rings appear like closely-spaced, parallel bands. The rays appear like scattered blotches. This plane is very beautiful in hardwoods such as oak. Since relatively few, large, perfect, quarter-sawed boards can be cut from a log, they are more expensive. Because the dense, dark summer bands (annual rings) are closely spaced, this plane is also more wear-resistant.

Radial plane of ponderosa pine (Pinus ponderosa) showing the closely-spaced, parallel annual rings. This is also called a quarter-sawed board. It is more resistant to wear because the dense summer bands are very close together.

All three planes are shown in the following 3-dimensional illustration:

Three planes of wood: A. Transverse, B. Tangential, and C. Radial

A block of oak wood showing the tangential plane (T) and the radial plane (R). The parallel lines on the radial side are annual rings. The blotches of cells at right angles to the annual rings are rays (ribbonlike aggregations of cells extending radially through the xylem tissue).

Cross section of a pair of oak bookends showing the prominent rays. Each ray (blue X in photo) starts in the center and extends radially like the spokes of a wheel. Rays are composed of bands of thin-walled parenchyma cells that conduct nutrients and water laterally in a stem. Because their walls are not heavily lignified like the surrounding xylem cells, ray cells disintegrate in dead wood and often result in radial splits in the wood. One notable comment about these bookends is that they are made of petrified oak. Millions of years ago, the original cells in this trunk were completely replaced by minerals. This piece of oak has literally turned into stone.

The following illustration shows two ways that logs are cut longitudinally (lengthwise) at the saw mill. Diagram "A" shows a log that is cut tangentially into boards. The tangential cut is also called slab-cut or plain-sawed (plainsawn) lumber. Diagram "B" shows a log that is cut radially into boards. The radial cut is also called quarter-sawed (quartersawn) lumber because the log is actually cut into quarters.

Longitudinal sections of a log: A. Tangential and B. Radial

Knots are the bases of lateral branches (limbs) that have become completely enveloped by the growth of new xylem tissue produced by the cambium layer of the trunk. Knotty pine boards from lodgepole pine (Pinus contorta) and other species make attractive wall paneling and cabinetry.

Knotty pine board, possibly from lodgepole pine (Pinus contorta).

The thick, outer layer of bark on a tree trunk is composed of suberized cork cells which contain a waxy, waterproof coating called suberin. The thick cork layer or phellem is produced by a special meristematic layer (outside of the cambium and phloem layers) called the phellogen. The cork layer becomes deeply fissured as the trunk expands in girth. Special openings in the bark layer called lenticels allow for gas exchange. Lenticels are very prominent in some stems. In Portugal, the thick cork layer is peeled off of the cork oaks (Quercus suber). The phloem layer is not destroyed, and the cork can be harvested repeatedly from the trees. In most trees, peeling off the bark will kill the tree because the vital phloem layer is also ripped away and destroyed. Removing a ring of bark from the trunk of a tree or shrub is called girdling. Cork is still the best stopper for perishable beverages, such as fine wines, because other synthetic polymers may affect the flavor and quality of the wine. For obvious reasons, bottle corks are generally cut at right angles to the lenticels.

Cross section of a limb from the cork oak (Quercus suber), showing the thick layer of cork bark. The bark is peeled off the trunks to obtain the commercial source of cork. Bottle corks are generally cut so that the lenticels are at right angle to the cork (see nail through lenticel in photo).

A bowl made from the cork oak (Quercus suber), showing the thick layer of cork bark. Made by Tom Cummings of the San Diego County Wood Carvers Guild.

Petrified Redwood In Yellowstone National Park

Lamar River Valley with Specimen Ridge in the distance, Yellowstone National Park. This range contains well-preserved petrified trees that are still in their original upright posdition. The trees in the foreground are narrow-leaf cottonwood (Populus angustifolia).

Left: An upright, petrified redwood in Yellowstone National Park. Volcanic eruptions in this region during the Eocene epoch (50 million years ago) triggered massive landslides into mountain and valley streams. The mixture of ash, water and sand buried entire forests, including redwood trees very similar to those of coastal California. Before the wood decayed, silca from the volcanic mud flow replaced the cell contents (lumens), literally creating forests of stone. Unlike other petrified woods that are completely replaced with minerals, the petrified wood of this region has lignified cell walls of the original xylem tissue that are still intact. Right: A 50 million-year-old piece of petrified wood (possibly redwood) from nearby Lamar Valley compared with a piece of recent dead wood. Both are radial sections with parallel annual rings. Can you determine which piece is petrified?

Petrified Wood Near Cameron, Arizona

Petrified wood fragments in northern Arizona near Cameron. These are probably from the Chinle Formation dating back to the Triassic Period, approximately 220 million years ago.

End view of one of the above wood fragments in its natural state. It was not altered (sectioned or polished). The fine grain structure shows perfectly preserved tracheids viewed in a transverse (cross section) plane. This wood predates the larger vessels characteristic of angiosperms. Photo taken with a hand held Nikon D-90 and SB 400 Flash using a 60mm Micro Nikkor AF-S F/2.8G ED Macro Lens. Camera settings: ISO 200, F22, 1/200. The high density, thick-walled tracheids and parallel rays resemble some of the images in the Online Publication by Dr. Rodney A. Savidge (2007), particularly the genera Pullisilvaxylon and Chinleoxylon (previously listed under Araucarioxylon arizonicum).

Note: The average diameter of one tracheid cell lumen in cross section is about 30-40 µm. The resolving power of an unaided human eye with 20-20 vision is about 70 µm. Therefore, the cellular detail in above wood fragment is invisible to the naked eye.

   See A Taxonomic Problem With Araucarioxylon arizonicum   

  • Savidge, R.A. 2007. "Wood Anatomy of Late Triassic Trees in Petrified Forest National Park, Arizona, USA, in Relation to Araucarioxylon arizonicum Knowlton, 1889." Bulletin of Geosciences 82 (4): 301-328.

Cross section of petrified wood fragment from Cameron, Arizona compared with a modern pine stem. Both stem cross sections show two spring growth periods with larger tracheids separated by a narrower summer growth band with smaller tracheids. The petrified wood dates back to the Triassic Period, approximately 220 million years ago.


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