BENNETTITALES 



73 



stamens is always shown by a distinct "shoulder" or "hyjjogynous 

 disk" on the receptacle between the enveloping sterile bracts and 

 the seed-bearing structures. This interpretation is confirmed by the 

 fact that in strobili 

 containing mature 

 stamens the ovulate 

 structures are always 

 present in a very young 

 stage. It may be pre- 

 sumed that when the 

 stamens were mature, 

 the ovules were ready 

 for pollination. The 

 period between ])ol- 

 lination and the ap- 

 pearance of a fully 

 formed embryo is 

 usually long enough 

 among gymnosperms 

 to account for the 

 absence of stamens 

 from the strobili of 

 Bennettitales bearing 

 mature seeds. It may 

 be unsafe to assume, 

 as yet, that there were 

 few Bennettitales with 

 monosporangiate stro- 

 bili, but at present the 

 bisporangiate strobilus 

 is one of the out- 

 standing features of 

 the group. 



The general structure of the strobilus is as follows: The tip of 

 the axillary dwarf branch or peduncle is expanded into a hemispherical 

 receptacle or is continued more or less as a thick axial receptacle. 

 Upon this receptacle three sets of members are developed in spiral 



Fig. 59. — Cycadeoidea: restoration of an unexpandud 

 strobilus, with part of the enveloping hairy bracts re- 

 moved; 18 folded fronds are shown as in C. dacotensis; 

 about natural size, or not nearly so large as various ex- 

 amples. — Figure and description after Wielaxu (22). 



