. Morphology of gymnosperms. Gymnosperms; Plant morphology. 132 MORPHOLOGY OF GYMNOSPERMS 3. The gametophytes THE FEMALE GAMETOPHYTE The megaspore is the first cell of the female gametophyte. It germinates immediately, a prolonged period of free nuclear division being accompanied by a much greater increase in the size of the embryo sac than in the quantity of cytoplasm, resulting in the formation of a large central vacuole which presses the cytoplasm with its nuclei against the megaspore mem- brane. Although cell- formation has not been studied among cycads, it is evident that it begins at the
. Morphology of gymnosperms. Gymnosperms; Plant morphology. 132 MORPHOLOGY OF GYMNOSPERMS 3. The gametophytes THE FEMALE GAMETOPHYTE The megaspore is the first cell of the female gametophyte. It germinates immediately, a prolonged period of free nuclear division being accompanied by a much greater increase in the size of the embryo sac than in the quantity of cytoplasm, resulting in the formation of a large central vacuole which presses the cytoplasm with its nuclei against the megaspore mem- brane. Although cell- formation has not been studied among cycads, it is evident that it begins at the periphery of the embryo sac and advances toward the center, as has been described for other gymnosperms (figs. 132- 134). From the figure it will be seen that cell-divi- sion is more vigorous at the periphery. Even after the gametophyte has become cellular throughout, sugar is the principal food content of the cell; but as the ovule approaches its full size, starch becomes increas- ingly abundant and in the mature seed it occupies almost the entire cell. The cells of the outermost layer are sharply differentiated from the rest, not only by their smaller size, but also by their almost. Figs. 132-134.—Dioon edule: stages in the development of the endosperm; fig. 132, small portion of periphery of endosperm some time after it has become cellular throughout; fig. 133, still later; the two rows (a) have come from a single row (like a) of the preceding figure; fig. 134, mature endo- sperm, showing thick megaspore membrane at the left, the outer row of cells with little or no starch, the next row with small starch grains, and the rest with large starch grains; the evenly shaded cell contains tannin; allX88.—^AfterChamberlain(46).. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Coulter, John Merle, 1851-1928;
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