SPERMATOPHYTA 



333 



Summary. The Cordaitales, a dominant Paleozoic group, seem not 

 to have been derived from the CycadofiHcales, although both groups may 

 have had a common origin. The stem is an endarch siphonostele. The 

 primitive features of the group, which they share with the Cycadofilicales 

 and the other cycadophytes, include a large pith, mesarch leaf bundles, 



V ii 



A B ' 



Fig. 282. Strobili of Cordaianthus, X8. A, longitudinal section of staminate strobilus, 

 showing sterile bracts and microsporophylls bearing terminal sporangia; B, longitudinal 

 section of ovulate strobilus, showing sterile bracts and two stalked axillary ovules. (After 

 Renault.) 



the ovule structure, and swimming sperms. The advanced features are 

 the branching habit, thick vascular cylinder, form of the leaves, and com- 

 pound ovulate strobili. The Cordaitales seem to have given rise to both 

 the Ginkgoales and Coniferales. 



5. Ginkgoales 



The Ginkgoales are represented by only one living species. Ginkgo 

 biloba, a native of western China. It is widely cultivated but virtually 

 unknown in the wild state. The order, probably derived from the Cordai- 

 tales during the Carboniferous, has been recognized as far back as the 

 Permian. Its members were most abundant during the Mesozoic, par- 

 ticularly in the Jurassic, when the order had almost a world-wide dis- 



