CORDAITALES AND GIXKGOALES 275 



494. The Conifer Ancestors of the Paleozoic period 

 (Order Cordaitales) were hirge trees 30 or more meters 

 in height, and bearing a dense crown of ])ranches and 

 hirge, paraHel-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. 



Fig. 151. — Cordaites. Fig. 152. — Ginkgo (staniinate 



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 biloha) 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 ])e 

 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. 



