The wide variety of technical terms utilized in describing the microscopic anatomy of tree thin sections is complex. This section contains a brief review of the most common terms encountered by visitors in the Molecular Expressions Tree Collection pages.

Fiber Tracheids - As the primary water-conducting elements in gymnosperms and seedless vascular plants, tracheids are elongated cells (average 1 millimeter long) that are closed at both ends and appear square in cross-section in the xylem tissue of trees. The thick, lignified walls are perforated so that water can flow from one tracheid to the next. The xylem of conifers (pine trees) and ferns only contain tracheids.

Heartwood - The central, woody core of a tree consisting of inactive tissue that has support as its only function. Instead of water or reserved nutrients, heartwood cells contain substances such as resins, tannic acid, dyes, oils, rubber, or other organic compounds that provide commercial use to the timber and chemical industries. As a tree ages, the heartwood increases in diameter and becomes denser and darker in color while the sapwood remains about the same thickness and color. Typically, spruce, pine and some flowering trees such as willows and poplars (known as "softwoods"), do not contain heartwood.

Heterocellular Ray System (or Heterogeneous Rays) - As a type of ray system in the wood of a tree, heterocellular rays are characterized by more than one type of cell, possibly including ray parenchyma (procumbent cells, square cells, and upright cells), sheath cells, and ray tracheids. Hardwood species, such as the oaks and the walnut trees, are comprised typically of two kinds of ray cells, procumbent and upright, while conifers (softwood tree species) such as pines, larches, and fir trees have rays consisting of ray parenchyma and various types of ray tracheids. The willows feature heterogeneous rays of square and upright marginal cells. The type of ray system serves as a diagnostic characteristic by wood anatomists to identify the trees of origin from their wood specimens.

Homocellular Ray System (or Homogeneous Rays) - As a type of ray system in the wood of a tree, homocellular rays are characterized by one type of cell. The alder, maple, and sycamore tree species have exclusively procumbent ray cells. The type of ray system is used as a diagnostic characteristic for identifying wood specimens to their tree of origin.

Libriform Fibers (or Xylary Fibers) - A class of fibers, composed of several cell types, which occurs within the complex vascular tissue system of a tree. Libriform fibers are very long, thin cells of the secondary xylem (or wood) that resemble the bark or phloem fibers of a tree and are characterized by simple pits. They may be referred to as xylary fibers (of tracheid origin) to distinguish them from similar looking extraxylary fibers that have a different evolutionary origin. Fibers with their lignin removed contribute the majority of the dry mass to a sheet of paper and they contribute to the characteristic strength, opacity, density, and porosity of the paper. Libriform fibers, typical of hardwood trees like oaks and hickories, are often 1 millimeter in length and 20 millimeters width as compared to the longer fibers of softwood species (conifers or pine trees) which are about 3 millimeters in length and broader at 20 to 35 millimeters width. The quality and quantity of fibers found in a tree depend on its species, its age, and its environmental conditions including the presence of shade, but the major component of wood from a deciduous (or hardwood) tree is libriform fiber. Although paper bag manufacturers seek the strength and resistance to tearing typical of softwood fibers, to create fine writing papers, it would be foolish to use 100 percent of the relatively large conifer fibers, resulting in paper that is too porous for ink and too rough. The libriform fibers of hardwoods are featured in high quality papers mixed with other fibers. Corrugated boxes are made of both an inner or flat layer of bleached softwood pulp to maximize resistance to compression failures and an interior corrugated or fluted part of high-yield hardwood fibers retaining a high lignin content to provide strength.

Metatracheal (or Apotracheal) - An older term used to describe the parenchyma of wood in which concentric bands of parenchyma develop independent of the vessels. Apotracheal has largely displaced the modifier metatracheal when describing wood characteristics. Metatracheal-diffuse (apotracheal-diffuse) parenchyma refers to single parenchyma strands or cells distributed irregularly among fibers or tracheids, when viewed in a cross-section of a tree's woody material. Metatracheal (or apotracheal) parenchyma is axial parenchyma independent of the pores or vessels and is a morphological character that distinguishes some birches and alders (partially diffuse xylem parenchyma) from their relatives such as oaks and chestnut trees (exclusively metatracheal parenchyma). In contrast, paratracheal parenchyma is axial parenchyma in which the cells are obviously associated with the vessels. In the aliform type of paratracheal parenchyma, cells extend tangentially from the vessels and are seen in cross section as lateral wing-like projections.

Parenchyma (also referred to as Ground Tissue or Soft Tissue) - Cells in a tissue of a tree that are concerned with function rather than structure. Parenchymal tissue, composing the bulk of all plant tissues, has specialized embedded cells that are characterized as large or small, thin-, medium- or thick-walled and usually contain a central vacuole. In plants, parenchyma cells perform the functions of storage (cells of the white potato and the pulp of fruits), assimilation (chloroplasts predominate in parenchymal-dominated leaves), or wound healing (resins and gums). In conifers, parenchyma cell arrangements form resin ducts or canals in the rays, usually at the end of each growth ring. Along with the rays, the parenchyma forms the food storage system of the tree. Parenchyma is often lighter in color than the rest of the wood and the pattern of its arrangement is used as an identifying characteristic for tree species.

Perforation Plates - The connecting wall ending between two tree vessels. As the tree ages and grows, the plate, originally without perforations, develops multiple openings. Used in tree and wood anatomy to distinguish species, a reticulate perforation plate has multiple perforations in a net pattern while a scalariform perforation plate has multiple, elongate and parallel perforations. Water and other tree fluids flow through the perforation plates, enabling the tree vessels to act as long, continuous transport tubes. In contrast, tracheids (although less efficient) have pits in their walls, but no perforation plates.

Pits (or Bordered Pits) - The openings in the cell wall for connection between tree cells consisting of a pit cavity and pit membrane. Pits allow the exchange of solutes in woody plants and are very porous, allowing the movement of water and ions from tracheid to tracheid. Bordered pits are typical for tracheids (gymnosperms and seedless vascular plants) while wood vessels are more typical of flowering plants (angiosperms). The balsam poplar is characterized by both simple and bordered pits.

Rays (also referred to as Pith Rays or Medullary Rays) - Rays are ribbons of cells aligned radially in a tree. Constructed of parenchyma cells arranged with their longitudinal axes oriented axially or radially, rays occur in wood as single or multiple layers (storied and seriate, respectively). The rays of angiosperm wood rarely contain tracheids and are larger than those in gymnosperm wood. Pith rays are typically viewed in the cross-section of a tree trunk running radially rather than in concentric circles.

Sapwood - Active, water-conducting tissue comprised of living cells. Sapwood is the youngest, thin outer part of a woody stem of a tree, adjacent to the bark and is softer and lighter in color than heartwood. As the tree grows and ages, the inner sapwood layers become heartwood and new sapwood is produced in the perimeter regions.

Scalariform - In wood and tree anatomy, scalariform is a term used to classify and describe vessel perforation plates, inter-vessel pits, and the secondary wall structure of the xylem vessels that are arranged in a ladder- or stair-like pattern. In general, scalariform perforation plates, typical of poplars, mistletoes, birch, and tulip trees, are considered to be more primitive traits than simple perforation plates (typical of oaks, elms, maples, apple trees and 80 percent of the world's tree species) or other patterns (annular, spiral, reticulate, or pitted). With the aging of the tree and the thickening of the xylem, the scalariform (or other perforation plate) pattern of lignification will develop in the wider, older vessels.

Seriate - A term derived from the Latin, meaning arranged in a series or succession, seriate is a characteristic pattern of rays in trees used to identify a type of wood in tree anatomy based on the number of rows of rays. As an example, apple tree wood is usually two-seriate (two rows) while highly valued wood from teak trees features a 4-seriate (or more) wide band of rays that are from one to six cells wide. Tracheids are also described as uni-, bi-, tri-, or multi-seriate (multiple rows).

Storied - When used to describe tree and wood anatomy, rays are classified as storied (or unstoried), based on if they are vertically stacked (or not). The American beech and American basswood are characterized as unstoried, as are most hardwood species while the white cedar and other softwood species (balsam fir, for example) typically reveal storied rays from one to thirty cells in height.

Tylosis (or Tyloses) - The outgrowth from a parenchyma cell through the pit cavity into a vessel, leading to its blockage. Usually, tylotic trees have restricted flow of water due to disease, adverse environmental conditions, or injury, plugging the xylem system and threatening the tree's survival. Tyloses are also utilized to characterize tree species in wood anatomy. For example, the American beech and black locust are two tree species characterized by tyloses in their heartwood. If a wood's permeability is low because of tyloses, it dries very slowly. It is also very difficult to treat wood with preservatives for use as outside lumber if it has abundant tyloses. For production of barrels, heavily utilized in the wine or whiskey industry, white oak (high number of tyloses) with its very low permeability is preferred to red oak (low number of tyloses). When wood is examined in transverse section, tyloses usually glisten.

Vasicentric - A type or arrangement of tracheids used to classify wood and trees. Vasicentric tracheids are adaptations for drought survival such as is typical of the woody flora of Southern California and Northern Mexico chaparral. Vasicentric means surrounding vessel elements with complete circles as opposed to aliform (surrounding the vessels but with wing-like extensions), confluent (wing-like extensions run together linking the vessels), and vascular tracheids. With larger diameters but shorter fibers, vasicentric tracheids contrast with vascular tracheids that appear to be more similar to latewood vessels. Oaks and eucalyptus trees are characterized by these thin-walled cells that surround vessels and have closed ends with lots of pits. In comparison, vascular tracheids, typical of elm and hackberry trees, are similar to narrow vessel elements, but have closed ends.

Vessels (also Wood Vessels, Xylem Vessels, Trachae, or Pores) - As the water-conducting elements of angiosperms, vessels are characterized by the water-filled tubes of the xylem of wood in hardwood tree species. Malpighi misnamed them as trachae in the seventeenth century on the ill-conceived notion that vessels were an important common element in plant and animal anatomy. Composed of numerous thick-walled, relatively large diameter cells, vessels are as long as the entire shoot in some plant species and are considered more efficient than tracheids in moving water. Strengthened with lignin, xylem vessels are non-living ducts and are deposited in spiral and ring patterns characteristic of the tree species, usually perforated by pits.

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