Rays
Xylem rays (wood rays) are sheets of parenchyma (sometimes sclerosed) extending radially inward from the cambium. In most woody plants, a few primary rays are extended by the cambium as secondary rays, while numerous other secondary rays are added by the
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Rays
6.1 Terminology, Definitions
Xylem rays (wood rays) are sheets ofparenchyma (sometimes sclerosed) extending radially inward from the cainbium. In most woody plants, a few primary rays are extended by the cambium as secondary rays, while numerous other secondary rays are added by the cambium. In some herbs or woody herblike plants, many of the primary rays may be extended by the cambium with few modifications and only a small number ofwood rays added by the cambium. Rays are ordinarily subdivided into uniseriate, those rays that are only one cell wide at most, and multiseriate rays, which are two or more cells wide at the widest point. Multiseriate rays may have uniseriate wings of varying lengths. The terms "biseriate" and "pluriseriate", although not mentioned by the IAW A Committee on Nomenclature (1964), are sometimes used for rays that are two cells wide and more than two cells wide, respectively. The height of a ray is considered to be the vertical distance from tip to tip as seen in tangential section; should the ray narrow to a single cell in width and widen again, the height is the entire distance. Ray width is the horizontal axis of the ray as seen in a tangential section; the width of a ray is usually measured at its widest point. Unisenate rays may be only a single cell in height. Ray cells that are vertically Ionger than wide are said to be upright (erect). Cells that are radially Ionger than wide are termed procumbent. Cells that are about as tall as wide are termed square. These characteristics are defined on the basis of cells as seen in a radial section, and can be applied securely only in a radial section. However, the readerwill note that these terms are often used when referring to tangential sections, as in Figs. 6.2-6.10 here. One can do this only by viewing a radial section and applying the information one obtains there about upright, square, and procumbent cells to what one sees in a tangential section. A ray cell taller than wide as seen in a tangential section may, in fact, be square or procumbent rather than upright when one sees it in a radial section- or the reverse (a tile cell is a good example of the reverse). Upright cells can be so tall that in a tangential section they simulate a libriform fiber; in a radial section, the upright cells will be more obviously parts of rays, and will have blunt rather than tapered ends, so that they do not look like fibers, and with this knowledge, an observer can more readily distinguish upright ray cells from libriform fibers in a tangential section. If a ray has only procumbent cells, it is said to be homogeneous; the term homocellular is used by some authors for this condition. If a ray has square and/ or upright cells in addition to procumbent cells, it is said tobe heterogeneaus (heterocellular). The tendency to use the terms homocellular and heterocellular represents an effort to refer to cellular homogeneity or heterogeneity of rays without S. Carlquist, Comparative Wood Anatomy © Springer-Verlag Berlin Heidelberg 1988
Ray Dimensions
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