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. 
ih a 
Fie. 151.—Cordaites. Fie. 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 survivingspecies 
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. 
