. Un manuel de botanique pour les collèges et universités ... La botanique. 683 tiges l'arrangement des éléments. mestome Le hadrome - dans les tiges est généralement dans le leptome. Dans dicotyls mestome les brins sont disposés dans un cylindre brisé (6g. S41), qui peut devenir plus tard un vérin complet grâce à l'activité du cambium, tandis qu'en monocotyls les faisceaux sont dispersés, mais plus abondants hors- ward (fig. 550). La plus courante est l'arrangement de la tige, la garantie d'être hadrome leptome et côte à côte sur le même rayon, le plus leptome (figs. 541, 550) ; dans Cucwbita bicollater est l'arrangement

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. A textbook of botany for colleges and universities ... Botany. STEMS 683 The arrangement of the mestome elements. — In stems the hadrome generally is within the leptome. In dicotyls the mestome strands are arranged in a broken cylinder (6g. S41), which later may become a complete cylinder through cambium activity, while in monocotyls the bundles are scattered, though more abundant out- ward (fig. 550). The most common stem arrangement is collateral, the leptome and hadrome being side by side on the same radius, the leptome outermost (figs. 541, 550); in Cucwbita the arrangement is bicollateral, there being a leptome strand inward from the hadrome, as well as outward. In some plants, notably the pterido- phytes, the arrangement of the mestome elements is concentric, the leptome commonly forming a cylinder about the hadrome {hadrocentric arrangement, fig. loio), although there are many cases, as in monocotyl rhizomes, where the leptome is surrounded by hadrome {leptocentric or amphi- vasal arrangement, fig. 551). In young roots there are alternating plates of hadrome and leptome in the vascular cylinder {radial arrange- ment, fig. 555); if there are three xylem rays alternating with three phloem rays, the root is called triarch, while such terms as tetrarch, pentarch, hexarch, and polyarch mean respec- tively, four, five, six, and many rays of both xylem and phloem. In the col- lateral bundles of leaves the hadrome is uppermost, even in ferns, in spite of the hadrocentric arrangement in the stems. Sheaths encircling the mestome. — In most but not in all cases, there are one or two sheaths or layers of cells surrounding the vascular tract. The inner sheath, regarded as the outermost layer of the vascular region, is known as the pericycle or pericambium (also as the parenchyma sheath or phloem sheath), and commonly is made up of delicate parenchyma cells (fig. 555). Outside the pericycle, and regarded as the innermost layer of the cortex, is the endodermis (also k

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