﻿3 68 



BOTANICAL GAZETTE 



Second free nuclear period. — After the migration of nuclei, 

 there is a second period of simultaneous free nuclear division at 

 the base of the proembryo, but the nuclei above do not divide 

 (fig. 19). The mitotic figures of this division were observed in 

 only two cases, but in two others there was evidence that such a 

 division had recently occurred. Since so few cases were observed, 

 the extent of this free nuclear period could not be determined, but 

 early stages in the cellular period would indicate that there are 

 probably only two or three simultaneous mitoses before wall 

 formation begins. The cause of this second period is probably the 

 dense basal accumulation of cytoplasm during the migration of 

 nuclei. The embryo, the suspensor, and some cells which remain 

 within the limits of the egg are all formed from cells resulting from 

 the second free nuclear period, the nuclei and cytoplasm above 

 being resorbed by the growing embryo. 



Ikeno (3), in his classic account of Cycas revoluta, described 

 free nuclear mitoses at the base of the proembryo, with amitotic 

 divisions above, but did not give any further description of the 

 embryogeny. His investigation dealt chiefly with the pollen tube 

 structures and oogenesis. Treub (i), who had previously made 

 a study of Cycas circinalis, did not mention such a stage. 



Formation or walls.— At the close of the second free nuclear 

 period, with the last simultaneous mitosis, there is a simultaneous 

 formation of cell walls (fig. 20). At the middle of the cellular 

 region there are about three layers of cells with cellulose walls, 

 and at the edges only one layer. Above these cells with cellulose 

 walls there is a segmentation of the cytoplasm, but the walls do 

 not reach the cellulose stage and are weak and evanescent. Within 

 the cellular region, mitoses are no longer simultaneous, and each 

 nuclear division is followed by the formation of a wall. The region 

 soon becomes sharply marked off from the cytoplasm above (fig- 

 21). Later, the peripheral walls of the cells bordering upon the 

 cytoplasm become thickened, so that the cellular region is still 

 more sharply denned (fig. 23). The thickening is mucilaginous. 

 The free nuclei in the cytoplasm may undergo occasional divisions, 

 giving rise to numerous small nuclei (fig. 21), while the cytoplasm 

 bordering upon the cellular region becomes denser, losing more or 



