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Journal Article

Zonal Structure of the Shoot Apex of Dioon edule Lindl

Adriance S. Foster
American Journal of Botany
Vol. 28, No. 7 (Jul., 1941), pp. 557-564
Stable URL: http://www.jstor.org/stable/2437002
Page Count: 8

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Topics: Mother cells, Plant cells, Meristems, Cell lines, Plant growth, Cell division, Cell growth, Starches, Botany, Pith
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Abstract

This paper describes the results of a study of the zonal structure of the shoot apex of Dioon edule. The material consisted of terminal buds of large plants collected near Chavarillo, Mexico, as well as of several lateral buds obtained from the greenhouse collection of cycads of the University of Chicago. The shoot apex is dome-shaped and ranges from 1,600 to approximately 1,950 microns in diameter. Four principal tissue zones are recognized. The initiation zone, which appears as a cap of tissue at the summit of the apex, functions as the ultimate source of origin of all cells. It consists of a large number of surface cells and their periclinal derivatives which, in longisectional view, are arranged in vertical-oblique tiers. The center of the apex is occupied by the well-defined zone of central mother cells, appearing as an obovoid mass of enlarged, irregularly-arranged cells with thickened walls. In this tissue zone which is derived from the inner portion of the initiation zone, growth in volume appears to predominate. A characteristic renewal in the intensity of cell division occurs at the sides and base of this zone, producing respectively the inner portion of the zone of peripheral tissue and the zone of rib meristem. As new cell-groups are added from the central mother cells to these two zones, the thickened areas of the common "mother wall" become stretched and decrease markedly in thickness. The surface cells on the flanks of the apex do not constitute a tunica layer but by means of frequent periclinal divisions, contribute new cells to the outer portion of the zone of peripheral tissue. Leaf primordia, the cortex and the cambium-like zone responsible for the outward elevation of the crowded foliar organs and the development of the leaf-trace "girdles," are produced from the peripheral tissue zone. The broad pith of the shoot axis originates exclusively from the rib meristem. The many structural similarities exhibited by the shoot apices of D. edule, Cycas revoluta and Zamia are discussed. A significant contrast with Cycas is furnished by the more clearly delimited character of the zone of central mother cells in Dioon and Zamia.

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