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 Some unpublished 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 Treub (13). 



