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Journal Article

Zonal Structure and Growth of the Shoot Apex in Microcycas calocoma (Miq.) A. DC.

Adriance S. Foster
American Journal of Botany
Vol. 30, No. 1 (Jan., 1943), pp. 56-73
Stable URL: http://www.jstor.org/stable/2437394
Page Count: 18
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Zonal Structure and Growth of the Shoot Apex in Microcycas calocoma (Miq.) A. DC.
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Abstract

This paper describes the results of a study of the zonal structure of the shoot apex of Microcycas calocoma. The material consisted of one- and two-year old seedlings and the large buds of lateral branches of the tree. All collections were made from native colonies of this cycad growing in the province of Pinar del Rio, Cuba. The shoot apex of large buds is mound-like or dome-shaped and ranges in diameter between 0.5 and 2.0 mm., depending somewhat upon the size of the bud. The apex of seedlings is much smaller (300-400 microns in diameter). Four main tissue-zones occur in the shoot apex of the large buds. The initiation zone, which occupies the broad summit and flanks of the apex, consists of the initiating surface cells and their periclinal derivatives. The latter form prominent tiers of cells which "radiate" in a distinctive fan-like manner towards the contour of the apex. Growth in this zone occurs predominantly from the surface towards the center of the apex. The zone of central mother cells occupies the center of the apex and in longisectional view consists of files of greatly enlarged, highly vacuolated cells. The lateral walls of these cells are noticeably thickened and primary pit-fields are abundant. This zone originates from the inner cells of the initiation zone, and its growth occurs chiefly by cell extension in the vertical direction. The zone of peripheral tissue surrounds the core of central mother cells and consists of two portions viz.: (1) an outer region derived from the sides of the initiation zone and composed of small "undifferentiated" cells-this subzone, in transectional view, extends as a series of "lobes" between the bases of adjacent leaf primordia, and from this tissue new foliar structures arise in spiral sequence; and (2) an inner region composed of extending and dividing cells which originate from the longitudinal division of cells at the edge of the central mother cell zone. A portion of the cell lineages in the inner peripheral zone curve outwardly towards the adjacent foliar primordia and give rise to the cambium-like tissue from which the leaf traces develop; the remainder of this tissue-zone produces the outer region of the pith. The zone of rib meristem appears as a narrow and and shallow core of cells derived from the base of the central mother cell zone. Growth in the rib meristem is prevailingly in the vertical direction and the short files of cells, by radial expansion and division, ultimately give rise to the central region of the pith. A fundamentally similar zonal pattern occurs in the seedling apex. As in the seedling of Cycas revoluta, a distinct apical cell is absent. A discussion is given of the applicability of Reinke's (1880) neglected theory of "growth movements" to the interpretation of the peculiar zonation in the apex of Microcycas. Analogues with the distinctive fan-like arrangement of cells in the initiation zone are furnished by the thallus-tips of certain brown and red algae and by the development of the nucellus in ovules. The relation of growth to the essentially similar pattern of cell arrangement in many root caps is also discussed.

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