CORDAITALES AND GINKGOALES 275 



494. The Conifer Ancestors of the Paleozoic period 

 (Order Cordaitales) were large trees 30 or more meters 

 in height, and bearing a dense crown of branches and 

 large, parallel-veined leaves, sometimes a meter or so 

 in length. Microspore and megaspore cones are known, 

 and even the seeds have been preserved, and many of 

 their details of structure made out. 



Fia. 151. Cordaites. FIG. 152. Ginkgo (staminate 

 and ovulate). 



495. The Maidenhair Trees (Order Ginkgoales) re- 

 mind one in some respects of the preceding. They were 

 common in the Mesozoic period, but all are now extinct 

 excepting a single species (Ginkgo biloba) from eastern 

 Asia. They have parallel-veined, fan-shaped leaves, 

 and branching, woody stems. In the surviving species 

 the trees are dioecious. The bisporangiate micro- 

 sporophylls constitute a loose cone, while the mega- 

 sporophylls remind one of those of Cycas described 

 above. The seed integument becomes fleshy externally 

 and stony internally when mature. 



496. The Joint-firs (Order Gnetales), including several 

 rather widely separated families, should probably be 

 placed here, although their relationship is doubtful, 

 especially since they have non-ciliated sperms. Ephedra 

 is a widely distributed genus of green, branching, leafless 

 shrubs resembling Equisetum in appearance. Gnetum 

 includes tropical shrubs and trees with large pinnately 

 veined leaves; Tumboa (Welwitschia) occurs in tropical 

 west Africa. 



