150 



FUNDAMENTALS OF CYTOLOGY 



In conifers a similar course may be followed by the nucleus, but in some 

 cases the male cytoplasm remains with it and eventually surrounds the 

 fusion nucleus. As in angiosperms, the chromosomes of the two gametic 

 nuclei sometimes remain more or less distinct until after the first embryo- 

 nal mitosis. 



Embryogeny in gymnosperms, like female gametophj^te development, 

 includes an early coenocytic phase and a later cellular stage (Fig. 108, h, c). 

 The embryo grows, differentiates its cotyledons, and ceases development 

 before all the endosperm (female gametophyte tissue) has been digested 



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Fig. 109. — Spermatogenesis in a cycad (Dioon). 1, microspore. 2, pollen grain with 

 tube nucleus, generative cell, and prothallial cell. 3, germinating pollen grain. 4, genera- 

 tive cell has divided to form body cell and .stalk cell. 5, conspicuous blepharoplasts in 

 enlarged body cell in growing pollen tube. 6, two spermatozoids formed by division of 

 body cell; in each of tliem a spirally coiled blepharopla.st runs just beneath the cell mem- 

 brane and bears the cilia. 7, portion of section through spermatozoid, showing cilia growing 

 from the ribbon-shaped blepharoplast seen as cross sections of two of its coils; portion of 

 large nucleus at left. (No. 6 after H. J. Webber; others after C. J. Chamberlain.) 



away. In many conifers a remarkable process known as cleavage poly- 

 embryony occurs. During its period of elongation the original embrj-o 

 splits into a considerable number, one of which finally surA'ives. Neigh- 

 boring archegonia in the same gametophyte may also develop embryos, 

 but the mature seed usually has only one. About the embryo is the 

 remaining endosperm; outside of this are a trace of the nucellus and a 

 well-developed seed coat. Since the gymnosperm o^'ule is not enclosed 

 in an ovary, there is no true pericarp. 



Ferns. — Some of the relatives of the ferns produce spore quartets of 

 two morphological kinds which differ in sexual tendenc.y, but in tnie 

 ferns themselves the spores are of but one morphological type. They 

 are produced in sporangia on the leaves of the sporophyte. The meiotic 



