Microscopic Structure And 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.
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.
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.
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.
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.
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.
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.
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.
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.
All three planes are shown in the following 3-dimensional illustration:
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.
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.
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.