1 54 



MORPHOLOGY OF GYMNOSPERMS 



Stangeria paradoxa (90) show a few simultaneous free nuclear 

 divisions at the base of the proembryo before walls begin to appear, 

 while the upper portion shows amitotic divisions in Cycas and resting 

 free nuclei in Stangeria. The outer 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 that 

 no central vacuole is present even at a rather late period, when the 

 suspensor has begun to elongate (fig. 180). It is now known that 

 this is true of all the genera, even Cycas 

 showing no central vacuole during the early 

 embryogeny; but in all cases a vacuole ap- 

 pears sooner or later and finally the egg 

 becomes empty and preserves its outline 

 only on account of its very thick membrane. 

 In Dioon edule (70) and Stangeria para- 

 doxa (90), at the close of the free nuclear 

 period, walls appear simultaneously 

 throughout the entire egg, but the walls are 

 weak and evanescent, except at the base of 

 the proembryo, where they become per- 

 manent. In Macrozamia Moorei (83) and 

 Encephalartos Friderici-Guilielmi the seg- 

 mentation of the egg is also complete, 

 but all the walls persist until a central vacuole is formed by the 

 disorganization of the inner cells. In all cases permanent walls 

 finally appear at the base of the proembryo and these cells become 

 differentiated into three regions. The cells bordering upon the 

 central vacuole or upon the free nuclear region become more or 

 less haustorial; those immediately beneath elongate and form the 

 suspensor; while the cells at the tip remain meristematic and give 

 rise to the embryo proper. 



The suspensor is remarkably long (fig. 181), that of Dioon edule 

 reaching a length of 70 mm., but it does not branch or bear 

 more than one embryo. The suspensors start separately from the 



Fig. 186. — Dioon edule: 

 longitudinal section of ma- 

 ture seed, showing embryo 

 with two cotyledons and 

 plumule, endosperm, inner 

 fleshy layer represented 

 only by a line, stony layer 

 shaded, and outer fleshy 

 layer; natural size. — After 

 Chamberlain (70). 



