154 



MORPHOLOGY OF GYMN0SPER:MS 



simultaneous free nuclear divisions at the base of the proembryo 

 before walls begin to appear, and at the same time amitotic divisions 

 may take place in the upper portion. The peripheral walls of the 

 proembryo are a new formation, entirely independent of the wall 

 of the egg. At this point the work of Ikeno stops, and beyond it 

 Warming and Treub give very little detail. 



< In Zamia floridana Coulter and Chamberlain (33) found a 

 different condition, no central vacuole being present even at a rather 

 late period, when the suspensor has begun to elongate (fig. 180). 



Walls begin to appear at the base of the 

 proembryo, just as in the beginning of 

 wall-formation in the endosperm of most 

 angiosperms. 



In Dioon edule (70), at the close of the 

 free nuclear period, walls appear simul- 

 taneously throughout the whole egg, but 

 the walls are weak and evanescent, except 

 at the base of the proembryo, where they 

 become permanent (figs. 183-185). Dioon rt- 

 sembles Zamia in having no central vacuole 

 during the proembryo stages, but at a very 

 late stage, like that shown in fig. 182, the 

 egg becomes empty and preserves its con- 

 tour only on account of its very thick 

 membrane. 

 After wall-formation has become established at the base of the 

 proembryo, the sequence seems to be about the same in all the forms 

 which have been studied. The cells bordering upon the large cen- 

 tral vacuole, as in Cycas, or upon the free nuclear area, as in Zamia 

 and Dioon, undergo little or no change in form, while the cells imme- 

 diately below them become greatly elongated, constituting the suspen- 

 sor (fig. 180). The cells at the tip remain meristematic, and from 

 this apical region the embryo proper is organized. 



The suspensor is massive and tortuous and is remarkably long (fig. 

 181), that of Dioon edule, when straightened out, frequently reaching a 

 length of 70 mm., but it has never been observed to branch or to bear 

 more than one embryo. The suspensors start separately from the 



Fig. 186. — Dioon edule: 

 longitudinal section of 

 mature seed, showing em- 

 bryo with two cotyled6ns 

 and plumule, endosperm, 

 inner fleshy layer repre- 

 sented only by a line, stony 

 layer shaded, and outer 

 fleshy layer; natural size. — 

 After Chamberlain (70). 



