2i6 GYMNOSPERMS 



It would seem that the Ginkgoales have come from the Cordai- 

 tales, or that both groups have come from some common ancestor. 



The swimming sperm is a Pteridophyte character, which has been 

 lost by all living seed plants except the cycads and Ginkgo. However, 

 some of the Coniferales seem to have lost the swimming habit 

 rather recently, geologically speaking. If Ginkgoales came from 

 Cordaitales, the Cordaitales must have had swimming sperms; for 

 such a character, once lost, could not be regained. 



It was once thought that the Cycadales came from the Bennetti- 

 tales, but such a derivation is regarded as impossible; and now both 

 groups are generally believed to owe their origin to the Cycadol'ili- 

 cales. Similarly, it seems quite possible that both Cordaitales and 

 Ginkgoales have a common ancestry. If reproductive organs could 

 be found associated with the Ginkgoales-like leaves of the Carbonif- 

 erous, the problem would be simplified. 



The extremely fernUke leaf of the Ginkgoales would favor a direct 

 origin from some heterosporous member of the Fihcales. 



An origin from the Cycadofilicales seems to be supported only by 

 the leaf gap. The profusely branching habit, the extensive develop- 

 ment of wood, with a comparatively small amount of pith and cortex 

 together with the simple leaf, indicate a Coniferophyte rather than 

 a Cycadophyte alliance. 



With the increasing interest in paleobotany in America and the 

 Orient, as well as in England and Europe, and with new fields for 

 collecting being discovered, it is reasonable to hope that material will 

 be discovered which will help to solve the difficult problems of re- 

 lationships and the evolution of those structures upon which theories 

 of relationship are based. 



