. Morphology of gymnosperms. Gymnosperms; Plant morphology. 152 MORPHOLOGY OF GYMNOSPERMS After fertilization there is a prolonged period of free nuclear divi- sion, in which the mitotic figures are intranuclear and the spindles very conspicuous (fig. 178). Since some have regarded the blepharoplast as a centrosome, these figures have been examined with great care, and it can be stated confidently that no centrosomes are concerned in their formation. The free nuclear divisions are simultaneous, just as in the germination of the megaspore or in the early development of endosperm in most angiosp
. Morphology of gymnosperms. Gymnosperms; Plant morphology. 152 MORPHOLOGY OF GYMNOSPERMS After fertilization there is a prolonged period of free nuclear divi- sion, in which the mitotic figures are intranuclear and the spindles very conspicuous (fig. 178). Since some have regarded the blepharoplast as a centrosome, these figures have been examined with great care, and it can be stated confidently that no centrosomes are concerned in their formation. The free nuclear divisions are simultaneous, just as in the germination of the megaspore or in the early development of endosperm in most angiosperms. The number of successive free nuclear divisions in Zamia floridana (33) was estimated as eight, giving 256 free nuclei before any walls begin to ap- pear. In Cycas revoluia no estimate was made, but there are certainly as many as 256 nuclei and very probably more. In Dioon edule (70) there are at least nine successive divisions, giving 512 nuclei, but the divisions become irregular and not perfectly simultaneous; hence one cannot assume that if the number is over 512 it will be 1024. In Cycas circinalis, according to Treub (13), a large central vacuole appears and crowds all the nuclei to the periphery, so that they form a parietal layer (fig. 182). In Cycas revoluta Ikeno (27) gives a different account of the formation of the central vacuole, stating that a large number of small vacuoles are formed, and that the cytoplasm of the central portion of the proembryo with its contained nuclei then disorganizes, leaving a parietal layer of cytoplasm and nuclei, while a single large vacuole occupies the center. Some unpublished observations make it evident that both these accounts need revision; but it is easily seen that the proembryo becomes a sac, somewhat thickened at the base and with the wall composed of one or two layers. After this stage has been reached, Cycas revoluta (27) and. Fig. 182.—Cycas circi- nalis: beginning of wall- formation in proembryo; X2S.—After Treu
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