302 



BOTANICAL GAZETTE [may 



prothallium, which indicate that a single superficial cell has grown 

 to a considerable size in a young developing cushion region. The 

 remarkable increase in size and the characteristic contents made 

 these cells very conspicuous. Sooner or later, from one of these 

 superficial cells an apical cell is cut off by an oblique wall, and becomes 

 the growing point of a leaf. Sometimes an apical cell appears very 

 early, as shown in figs. 20 and 21. During these mitoses, the number 

 of chromosomes is always 64 or 66. 



While this process is in progress in the superficial region, cells 

 beneath divide in various directions. Mitoses, of course, occur in 

 various parts during the growth of the prothallia, but they occur 

 more rapidly in the interior region which borders the group of 

 superficial cells whose formation was described above. 



Some of the characteristic features that distinguish these interior 

 cells from the other prothallial cells are as follows: they are con- 

 siderably smaller, probably due to their rapid successive divisions; 

 the nucleus, in spite of the smallness of the cell, is large ; the cyto- 

 plasmic contents are abundant and the plastids are small and few 

 at this stage. 



The mitoses in connection with the formation of this group of 

 interior cells were studied. The general aspect of the mitotic figures 

 in successive stages and the behavior of the chromosomes in these 

 mitoses were similar to those of typical mitosis in the vegetative 

 cells of normal prothallia. It was interesting to notice, however, 

 that the narrowness of the cell cavity and the largeness of the nucleus 

 showed a remarkable resemblance to the condition observed during 

 the development of normal embryos later than the 32-celled stage 

 (figs. 22-2Q). In telophase the number of chromosomes was always 

 64 or 66 (fig. 27) . 



The mitoses in the groups of superficial and interior cells con- 

 tinue as described above, and there is formed a structure which is 

 in direct connection with the prothallial cells, the structure that later 

 becomes an independent sporophyte. Figs. 28 and 28a represent 

 the structure of the sporophytic outgrowth at a certain stage in the 

 course of its development. One apical cell which is already differ- 

 entiated in the structure is not shown in this section. 



Thus the structure of the sporophytic outgrowth is brought about 



