COOTPBRALES 101 



to the long axis of the oospore, a tier of four completely walled 

 cells thus being cut off below (Fig. 76, D), the upper nuclei 

 still being freely exposed above to the partially segmented 

 cytoplasm of the spore (Fig. 76, E), and increasing very much 

 in size. The cells of the single completely walled tier then 

 divide simultaneously, and two tiers are organized (Fig. 76, F). 

 The process is again repeated by the lower tier, and the result 

 is three tiers of four cells each (Fig. 76, G). With the appear- 

 ance of this last tier the development of the proembryo ceases. 

 At this stage, therefore, there are three complete tiers, with 

 four cells in each tier, and above these completely walled cells 

 there are the four nuclei separated from one another by walls, 

 but freely exposed above to the food supply of the spore. By 

 their size and generally active appearance it would seem that 

 they must continue functional for some time in connection with 

 nutritive work. 



It will be noted that the proembryo in Conifers differs from 

 that in Cycads in the fact that free nuclear division is much 

 restricted, resulting usually in a group of four nuclei rather than 

 a great number; that as a result there is no development of 

 tissue in a parietal position; that the tissue developed at the 

 base of the oospore is more definite and limited as to number of 

 cells and their functions. At the same time, the evidence seems 

 clear that the proembryo of Conifers must have been derived 

 from just such a condition as is shown by the proembryo of 

 Cycads and Ginkgo. 



While this account probably applies in outline to Conifers 

 in general, there are differences in certain details which are 

 suggestive. The outline which seems to apply to all Conifers 

 is as follows: the development of free nuclei from the fusion 

 nucleus, the placing of these nuclei in a plane at the bottom of 

 the oospore, the appearance of incomplete walls separating the 

 nuclei from one another but permitting their free exposure to 

 the nutritive mass above, the development from this plate of 

 nuclei of three tiers of completely walled cells, the development 

 of the embryo from the lowest tier of cells, the development of 

 the suspensor from the middle tier, and the retention of the 

 uppermost tier in the oospore. 



The details which differ from those given in the above ac- 

 count for Pinus are as follows : Recently Arnoldi ^* has de- 



