CYCADALES 



lOS 



fusion of its cotyledons. They 

 arise as two distinct organs, 

 but in the older embryos the 

 fusion is so complete that serial 

 sections from the apex down- 

 ward almost to the apex of 

 the plumule do not reveal the 

 characteristic "seam," in many 

 cases the abutting epidermal 

 layers having disappeared. As 

 in Ceratozamia, there is no 

 "resting stage" of the embryo, 

 which emerges from the testa 

 as soon as it is formed. In 

 Ceratozamia, Dioon, Micro- 

 cycas, and Zamia, and pre- 

 sumably in other cycads, the 

 root is much delayed in its 

 development, not being formed 

 when the base of the embryo 

 ruptures the testa and exposes 

 a small brown disk of dis- 

 organized tissue, which repre- 

 sents the remains of the once 

 extensive coleorhiza, and 

 through which the root may 

 not penetrate until the soil is 

 reached or even until after the 

 exit of the plumule. 



An interesting deviation is 

 shown in Microcycas by the 

 vascular cylinder of the tran- 

 sition region, which is not a 

 protostelic plate, as in the pre- 

 ceding cases, but is siphono- 

 stelic from the beginning (fig. 

 82). The four cotyledonary 

 strands remain distinct, and 



Fig. 81. — Dioon edule: semi-diagram- 

 matic reconstruction of part of vascular system 

 of embryo, to show especially the girdling; 

 Zi, li, traces of first and second leaves (£', L') ; 

 vp, vascular plate; px, protoxylem groups; 

 a, xylem elements continuing from proto- 

 xylem groups of plate to form, the protoxylem 

 of the primary root. — -Mter Thiessen (62). 



