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Journal Article

Further Studies on Zonal Structure and Growth of the Shoot Apex of Cycas Revoluta Thunb.

Adriance S. Foster
American Journal of Botany
Vol. 27, No. 7 (Jul., 1940), pp. 487-501
Stable URL: http://www.jstor.org/stable/2437084
Page Count: 15

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Topics: Mother cells, Cell lines, Plant cells, Meristems, Plant growth, Diameters, Cell growth, Plants, Botany, Pith
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Abstract

A study has been made of the shoot apex of large specimens of Cycas revoluta Thunb. The apices of plants of unknown sex as well as of microsporangiate and megasporangiate individuals furnished the material for investigation. The unusually massive apex ranges in diameter from 2-3.5 millimeters. These dimensions are far greater than those recorded for any vascular plant. In form, the apex varies from a low mound or dome to a scarcely elevated, flat-topped plateau. Three major tissue zones are distinguishable in the shoot apex. Zone I, which is the ultimate point of origin of all tissue, consists of a shallow cap of thin-walled, actively dividing cells occupying the broad summit of the apex. This zone includes an extensive series of morphologically identical surface cells and their most recent periclinal derivatives. Both anti-clinal and periclinal divisions occur throughout the cells of Zone I, and there is apparently no definable group of "apical initials." The massive core of central tissue represents Zone II, and its origin can be traced to the inner margins of Zone I. At these points, there occurs a gradual or abrupt transition from the small, thin-walled cells typical of Zone I to the enlarging, highly vacuolated, thicker walled cells characteristic of Zone II. Growth in this zone occurs by the irregular enlargement and seemingly random plane of division of "mother cells," the thickened walls of which assist in determining the form and extent of cell lineages. The possible application of Schuepp's (1926) concept of "Massiges Meristem" to the complex structure and growth of Zone II is briefly discussed. Zone III constitutes the peripheral tissue of the apex and is subdivided into an outer region (Zone IIIa) and an inner region (Zone IIIb). Zone IIIa originates directly from the surface cells at the edge of Zone I and consists of several irregular "layers" of very small, deeply stained cells, in which periclinal and anticlinal divisions occur. The most significant feature of this zone is its completely "undifferentiated" appearance. In contrast, the origin of Zone IIIb can be traced to "renewed" mitotic activity at the edges of Zone II. At such points, the cells divide anticlinally or obliquely with reference to the summit of the apex and produce filamentous cell groups which collectively form the tissue of Zone IIIb. The progressive basipetal differentiation of the tissue of Zone II gives rise to the pith of the shoot axis. Leaf primordia, the cortex, and apparently the provascular tissue are produced by derivatives of the peripheral tissue zone. The massive, structurally complex apex of C. revoluta is regarded as a morphologically "crude" type among living seed plants. A brief discussion of its relation to the Zamia, Ginkgo, and conifer "types" of apices is presented. The need for continued exploration of the zonal structure of the shoot apex in seed plants is emphasized.

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