types of wood cells include the following:

—bast a type of fiber that comes from the inner bark on numerous shrubs and trees and also herbs such as hemp and flax —note 1. when the inner bark is separated from the dark outer bark, and in some cases from the woody cores, bast yields long, strong fibers that are excellent for papermaking —note 2. inner bark bast fibers usually contain a higher percentage of hemicellulose than seed-hair fibers

—cambium a layer of living, meristematic cells between the wood (secondary xylem) and the innermost bark (secondary phloem) of a tree —note in each growing season, division of these cells adds a new layer of cells on the wood (xylem) already formed as well as a layer of inner bark (phloem) on the outer face of the cambium

—cellulose a straight-chain polymer built up of a large number of glucose anhydride molecules with the empirical formula of (C₆H₁₀O₅)_n —note cellulose is the principal chemical constituent of the cell secondary walls of higher plants and occurs (often with other components such as lignins, hemicelluloses, waxes, and gums) mainly as long, hollow chains called fibers —see isotropic

—companion cell a cell in the phloem intimately connected with a sieve-tube element and retaining the nucleus and dense cytoplasm —note companion cells may undergo some transverse or other divisions preceding their differentiation

—fiber a slender, threadlike component of wood, composed chiefly of cellulose —note 1. fibers differ from tracheids in being more slender, having simple pits and thicker walls in relation to diameter, and often retaining protoplasm —note 2. fibers are used in making pulp for paper and classified according to the part of the plant from which they are taken, i.e., stem, bast, leaf, seed-hair —see defibration, microfibril, tracheid —note 3. a gelatinous fiber has a more or less unlignified inner wall of viscuous appearance —see lignin

—hemicellulose a branched-chain polymer of cellulose made up of sugars —note 1. the chain length of hemicellulose is shorter than that of cellulose, and hemicelluloses are found in higher proportions in wood fibers and some bast fibers —note 2. compared with celluloses, hemicelluloses are less resistant to chemical and atmospheric degradation, bond more readily, and are more desirable for some types of papers

—holocellulose cellulose plus hemicellulose

—lignin a complex, high-molecular-weight polymer built on phenylpropane units, that occurs between individual cells and within cell walls, is intimately associated with cellulose, and serves to impart rigidity to the cell —note apparent lignin is lignin content as determined by a standard procedure, e.g., the Klason determination, without the removal of interfering extraneous substances —see sclerosis

—micelle a crystalline region in bundles of cellulose (microfibrils) separated longitudinally by amorphous regions —note the term micelle, originally proposed by Nägeli in 1885, is now often replaced by the somewhat different concept of crystallite, but the terms are not synonymous

—microfibril a cellulose bundle in which the cellulose molecules are mostly oriented parallel to the axis of the microfibril —note 1. the orientation of the microfibrils to the long axis of the fiber is called the microfibril (or micellar) angle; wide angles result in high longitudinal shrinkage of the fiber —note 2. because the S₂ layer of the cell wall is the widest layer, the microfibril angle of the S₂ layer has the greatest influence on longitudinal shrinkage of sawn boards

—parenchyma vertically or horizontally arranged living cells of the xylem that store food

—periderm the secondary tissue of stems or roots consisting of the phellem, phellogen, and phelloderm —see bark

—phellem cork tissue produced by the phellogen in a stem or root on the bark side (outside), nonliving at maturity, and having suberized walls —note the nontechnical name for phellem is cork; the bark of cork oaks, Quercus suber and Q. occidentalis, produce the commercial forms

—phelloderm tissue resembling cortical parenchyma produced by the phellogen in a stem or root on the pith side (inner side), radially arranged, and with a shape resembling that of phellogen cells

—phellogen secondary cambium that produces the periderm —synonym cork cambium

—phloem a layer of cells just inside the bark of plants that conducts food from the leaves to the stem and roots —note 1. the conducting elements of phloem are known as sieve cells but may also include companion cells, parenchyma cells, fibers, sclereids, and rays —note 2. primary phloem differentiates from derivatives of the apical meristem; secondary phloem is produced by the same vascular cambium that forms the secondary xylem

—pit an interruption or recess in the secondary cell wall through which fluids or gases pass from one cell to another —note 1. pits are usually paired in common walls of contiguous cells —note 2. bordered pits have secondary walls that overarch the pit membrane; simple pits do not have secondary walls —note 3. scalariform pits are elongated or lineal and are arranged in a ladderlike series

—pith the central core of a stem, branches, and some roots representing the first year of growth and consisting mainly of soft tissue

—pore —see vessel

—ray a type of wood structural element, one to many cells high but usually only one cell wide (uniseriate, ribbon-shaped), extending radially across the grain, that stores and distributes sap horizontally (radially) —note 1. fusiform rays have horizontal resin ducts at their centers; medullary rays (pith rays) are parenchyma tissues that provide for lateral transport of liquids and soluble material between the cortex and the pith —note 2. rays are usually readily visible on a quarter-sawn surface of a board

—sclereid a hard, often branched, strongly lignified, nonconducting cell that can occur anywhere in a plant but are most abundant in the phloem, fruits, and seeds

—sieve cell a nonlignified cell similar in shape to longitudinal tracheids of the xylem but somewhat shorter and having circular to oval areas of very thin walls called sieve areas —note the primary function of sieve cells is food conduction

—tracheid an elongate, spindle-shaped wood cell that loses protoplasm at maturity when becoming conductive —note tracheids account for 90 to 95 percent of softwood volume, much less in hardwoods

—tyloses saclike structures of protoplast that balloon through pits into the cavities of adjacent vessels of hardwoods and sometimes tracheids of softwoods —note 1. tyloses are commonly associated with heartwood formation and often partially or completely block vessels —note 2. the effect of tyloses on wood quality may be detrimental (making wood difficult to dry) or beneficial (as for barrels made to store liquids)

—vessel the principal vascular or water-conducting element of hardwood —note 1. vessel members differ from tracheids in having perforations, usually in the end walls —note 2. longitudinal series of vessel members constitute a vessel that can be many centimeters or even meters in length —note 3. vessels are sometimes visible as small holes on a cross-section of wood —synonym pore

—xylem the principal water-conducting tissue and the chief supporting system of higher plants, composed of tracheids (for conduction and support), vessels (for conduction), fibers (for support), and parenchyma (for food transport and storage) —note 1. xylem and associated phloem tissues constitute the conducting system of vascular plants —note 2. primary xylem consists of two forms: (a) protoxylem is formed while the organ is still elongating and generally includes cells with extensible secondary wall thickening and relatively few tracheids or vessel elements; (b) metaxylem is formed after growth in length is nearly completed and has cells mostly with nonextensible secondary walls accompanied by parenchyma and fibers —note 3. secondary xylem in most woody plants constitutes the bulk of the entire plant and is composed of two interpenetrating systems, radial (ray) and vertical (axial)This definition last updated 07/17/2008