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Journal Article

Structure and Growth of the Shoot Apex of Cycas revoluta

Adriance S. Foster
American Journal of Botany
Vol. 26, No. 6 (Jun., 1939), pp. 372-385
Stable URL: http://www.jstor.org/stable/2436837
Page Count: 14
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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.
Structure and Growth of the Shoot Apex of Cycas revoluta
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Abstract

A detailed study has been made of the structure and growth of the shoot apex of seedlings and adventitious buds of Cycas revoluta Thunb. The anatomy of the terminal meristem of the seedling of Zamia floridana A.DC. is also briefly described. No evidence has been found of the existence of a "permanent" apical cell. On the contrary, all the tissue of the shoot apex in both Cycas and Zamia can be traced in ultimate origin to the activity of a single group of apical initial cells which divide anticlinally, periclinally, and obliquely without regular sequence. Although a large prismatic cell may appear to dominate for a time in this initial group, its individuality is eventually submerged as a result of irregular and variable planes of segmentation. The tissue derived from the apical initial group in Cycas is not uniform in structure but is segregated into two more or less clearly demarcated zones: (1) a prominent conical zone of central tissue which is completely surrounded by a mantle of deeply-stained peripheral tissue. These zones are characterized by significant differences in the growth, planes of division, arrangement, and relative size of the component cells. Pronounced cell-enlargement, followed by the formation of well-defined cell groups, are the distinctive histogenetic features of the central tissue in Cycas. When the direction of growth and enlargement of a given "mother cell" is approximately vertical, a linear series of daughter cells is produced. This type of cell enlargement and cell formation is particularly evident in the lower region of the central tissue, where the irregular filamentous cell groups collectively resemble a rib meristem. At any position in the central tissue, however, cells may enlarge more or less uniformly in all directions, and the resulting cell lineages are correspondingly difficult to interpret. The walls of the central cells, especially at the corners, are often prominently thickened. A preliminary study of these thickened areas in an adventitious bud of C. revoluta between crossed nicols showed that the outer lamellae are strongly birefringent (i.e., anisotropic), while the inner region appears dark (i.e., isotropic). When treated with chlor-zinc-iodide followed by 65 per cent H2SO4, the outer portion of the thickenings gave the blue color typical of cellulose. The peripheral tissue of the apex in Cycas is composed of smaller cells characterized by their "embryonic" appearance (i.e., absence of impressive enlargement) and frequent divisions. Judging from the cell net and the orientation of mitotic figures, the prevailing direction of growth in this tissue-zone is centrifugal. The surface cells retain a marked capacity for continued growth and division; collectively, they do not resemble in arrangement or plane of division a "dermatogen." Periclinal divisions in these surface cells are followed usually by the anticlinal division of one or both of the daughter cells. Often several successive periclinal divisions, followed by anticlines or obliquities in the derivatives, may occur. The radially-aligned cell groups thus formed are complex in structure and variable in size. Foliar structures are initiated at the base of the shoot apex by a localized acceleration of growth and cell division in the peripheral tissue, accompanied by frequent periclines in the surface cells. Periclines are also numerous in the adaxial surface cells of the cataphyll primordia, which, at least during early ontogeny, do not possess a typical dermatogen. An anatomical comparison of the shoot apices of Cycas revoluta and Ginkgo biloba reveals few points of resemblance, and the terminal meristems of these forms are regarded as distinct "types" among living vascular plants. The closest structural resemblance with Cycas is exhibited by certain conifers such as Abies venusta.

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