4i6 MORPHOLOGY OF GYMNOSPERMS 



sperms. In this primitive group, in addition to microsporophylla 

 resembling fernlike leaves with abaxial synangia, there appeared 

 others that have been spoken of as the Crossotheca ("epaulet") and 

 the Calymmatotheca ("ciipule") types, and in all probability still 

 others will be discovered. All of these types were continued among 

 the coniferophytes, with varying details of minor importance. Among 

 some of the Gnetales either the sporophyll has become very reduced, 

 or it has become suppressed, so that the microsporangia are cauline; 

 but even in Welwitschia the old terminal synangium is evident. 



Among the cycadophytes, on the other hand, only what may be 

 regarded as the most ancient type of microsporophyll has been retained, 

 that is, the fern type with abaxial sporangia (often synangia). Among 

 the Bennettitales, there is so little departure from the old type that its 

 microsporophylls resemble pinnate fern leaves with abaxial synangia; 

 and even among the Cycadales the more or less leaflike microsporo- 

 phylls show the same character. If there is any tendency in the sta- 

 mens of this phylum worth noting, it is the tendency shown among the 

 cycads to reduce the sterile apex of the sporophyll to a more compact 

 peltate expansion. 



The microsporangium of gymnosperms is a very consistent struc- 

 ture, originating from the hypodermal layer of cells, and developing 

 a wall of several layers, the innermost one of which is differentiated 

 as the tapetum. The only general tendencies to be observed are 

 the gradual replacement of synangia by separate sporangia, and the 

 more rapid ehmination of all evidences of an annulus (in the general 

 sense). It is noteworthy that in both these particulars the cycado- 

 phytes, with their much more recent connection with the Cycado- 

 filicales, are far behind the coniferophytes. 



THE OVULE 



The origin of the ovule of gymnosperms remains in obscurity. 

 While the stamen and its sporangia repeat the corresponding structures 

 of ferns, the ovules of Cycadofilicales and of Cordaitales are so well 

 organized, even in the modern sense, that their cormection with the 

 sporangia or synangia of ferns is entirely a matter of inference. This 

 means a tremendous gap between the somewhat hypothetical Primo- 

 fihces, on the one hand, and the Cycadofilicales and Cordaitales, 



