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The Anomalous Root Structure of Cycas revoluta
American Journal of Botany
Vol. 23, No. 5 (May, 1936), pp. 336-340
Published by: Botanical Society of America, Inc.
Stable URL: http://www.jstor.org/stable/2436094
Page Count: 5
You can always find the topics here!Topics: Xylem, Tracheids, Blood transfusion, Parenchyma, Cambium, Secretory cells, Plant roots, Phloem, Plant cells, Gymnosperms
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The seven or eight protoxylem points at the stem end of a fleshy adventitious root of Cycas revoluta merge successively in pairs; at its tip the root, if sufficiently extended, possesses a diarch protostele. The fleshy character of the root is attributable to the thick cortex; the enlargement of parenchymatous cells, rather than the addition of new cells, is responsible for diametric growth. As a result of the enlargement of the stele during secondary growth, concentric wrinkles of crushed cells appear in the cortex at the stem end of the root, where it is crowded between other similar roots. The many-layered unbroken ring of pericvcle functions in most of the root as the principal storage organ, at least for starch. At an early stage the radially arranged phloem consists solely of phloem parenchyma; at a later stage, bast fibers mature. The protoxylem consists of elements whose secondary wall layers present tightly coiled spiral or scalariform thickenings. Soon after the maturation of protoxylem, a centripetal metaxylem develops, consisting of tracheids with both simple pits and well-developed bordered pits; but no root was examined that showed a completely solid protostele. Subsequently, erratically marked reticulate metaxylem matures mostly in a centrifugal direction to leave the protoxylem almost in the center of the compact xylem ray; the root, therefore, is mesarch. Scattered promiscuously throughout most of the tissues of the root occur secretory cells filled with granular or solid mucilage-like material. Cambium is differentiated from a relatively wide meristematic zone and gives rise inward to secondary xylem with bordered pits and outward to phloem parenchyma and bast fibers; not until a late stage is cambium differentiated from the pericycle outside each xylem ray. A cork cambium is differentiated early from hypodermal layers; in the oldest roots examined, periderm had not developed in the pericycle. The complexly reticulate metaxylem, mostly centrifugal in its development, parallels in ontogeny, if not in phylogeny, the origin of similar reticulate elements of transfusion tissue found in gymnosperm leaves; its survival evidently has depended upon its function as an accessory conducting tissue.
American Journal of Botany © 1936 Botanical Society of America, Inc.