346 MORPHOLOGY OF GYMNOSPERMS 



4. The embryo 



The two general stages in the development of the embryo of Pina- 

 ceae are repeated among Taxaceae, but there is more variation in 

 the character of the proembryo, and it more commonly fills the whole 

 egg. The fact that among Pinaceae the proembryo usually occupies 

 oniy a relatively small part of the basal region of the egg is not due 

 to the small number of cells, for these may be just as numerous as 

 among the Taxaceae, but is due to the large size of the egg at fertiliza- 

 tion. The proembryo of Taxaceae also differs in the lack of uniform- 

 ity of its tiers, the embryo-forming tier usually being represented by 

 a single cell, while the suspensor and "rosette" tiers may be repre- 

 sented by a considerable number of cells; a fact that is probably due 

 to the size and configuration of the egg. 



THE PROEMBRYO 



The development of the proembryo of Torreya taxifolia (loi) 

 may be used as an illustration. The division of the fusion nucleus 

 is followed immediately by a second division, and the four large free 

 nuclei almost fill the small egg (fig. 401). As a consequence, there is 

 no such movement of nuclei toward the base as occurs in larger eggs. 

 At this foiur-nucleate stage wall-formation occurs in the usual way by 

 connecting fibers, the four walled cells completely filling the egg. 

 Division of these cells continues until a proembryo of about twelve 

 cells is produced, arranged more or less definitely in three tiers, as 

 follows: five or six in the "rosette" tier, five or six in the suspensor 

 tier, and one in the embryo-forming tier (fig. 402). Later divisions 

 may result in a proembryo of eighteen cells. It is this twelve to eight- 

 een-celled proembryo that persists, without further change, until 

 the following spring. The relatively small egg, the very large nuclei, 

 and the early appearance of walls are probably accountable for the 

 tissue-filled egg. This account agrees w^ith that given for Torreya 

 calif ornica (99). 



In Cephalotaxus fertilization occurs later in the history of the game- 

 tophyte and the eggs are larger. In the two species investigated, C. 

 Fortunei (124) and C. drupacea (130), four free nuclei are formed and 

 then assume the basal position, in which nuclear division continues 



