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The Cellular Proembryo of Zamia and its Cap Cells
George S. Bryan
American Journal of Botany
Vol. 39, No. 7 (Jul., 1952), pp. 433-443
Published by: Botanical Society of America, Inc.
Stable URL: http://www.jstor.org/stable/2438326
Page Count: 11
You can always find the topics here!Topics: Embryos, Cell walls, Cellular differentiation, Embryonic cells, Plant cells, Cell growth, Meristems, Cell nucleus, Cytoplasm, Gametophytes
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Following the free nuclear development of the proembryo of Zamia, a cellular proembryo is formed in the base of the old archegonium. Scattered elsewhere in the old archegonium are a variable number of nuclei that were not incorporated in the cellular proembryo. The cellular proembryo does not differentiate into embryo and suspensor but into four regions: a basal meristem whose outer layer of cells eventually forms a conspicuous cap; immediately above the meristem elongating cells which become the first cells of the suspensor; and, above the first suspensor cells and extending part way up the sides of the archegonium, buffer cells. The number of cap cells is variable, ranging from about 25-80 cells. The cap cells, in their growth and development, and even after reaching maturity, may divide periclinally. The mature cap cells possess densely granular cytoplasm, large nuclei and thick cell walls. The outer ends of the walls are particularly thickened. The posterior part of the meristem continually adds elongating cells to the suspensor which eventually becomes coiled and twisted. The pressure thus produced keeps the cap appressed to the base of the digested cavity in the endosperm. At about the time of, or soon after, the disintegration of the cap, the upper cells of the coiled portion of the suspensor collapse forming a tough, thread-like structure; and at the same time the meristematic region begins a marked growth in breadth and in depth. Cap cells have not previously been reported for any of the cycads. On critical investigation caps will undoubtedly be found in at least some of the other genera. Like Araucaria, Agathis and Cephalotaxus, Zamia produces a deciduous cap. It is probable that 2 species of Podocarpus should be included in the list. The function of the cap is a matter of opinion. The hypothesis that deciduous caps may be relict structures is suggested.
American Journal of Botany © 1952 Botanical Society of America, Inc.