GEOLOGICAL HISTORY OF GYMNOSPERMS 35 



grains continued to grow after leaving the microsporangia and 

 showed a group of prothallial cells or antheridium which devel- 

 oped motile sperms directly and without the formation of a 

 pollen tube. The seeds were bilaterally symmetrical and usually 

 winged. 



Although we know Cordaites in detail, the impressions of 

 foliage indicate that it constituted a large and diversified genus. 

 Other types are known from fragments showing stem anatomy 

 {Pitys, Poroxylon, Mesoxylon, CalUxylon, Caenoxylon, Meso- 

 pitys, Parapitys, etc.) and it is quite possible that the Ginkgo 

 line diverged from somewhere in the Cordaitean plexus in the 

 late Paleozoic. 



Both the Pteridosperm and the Cordaites stocks are known 

 from the Devonian: Both were dominant groups that became 

 cosmopolitan in the Carboniferous : Both continue in the Permian 

 and neither is certainly known from the Mesozoic although a 

 few stragglers may have lingered on into that era. 



Three additional types of gymnosperms appear in the late 

 Paleozoic record but as our knowledge of both is based largely 

 upon impressions of the foliage little is known of their exact 

 affinities. These three groups are the Cycadophyta, the Arau- 

 cariaceae and the Ginkgoales, all of which became greatly dif- 

 ferentiated and common during the Mesozoic. The genus 

 Psygmophyllum of the Devonian Carboniferous and Permian is 

 supposed to represent the Ginkgoales as is also the genus Whittle- 

 seya of the Carboniferous. In the Permian a considerable num- 

 ber of Ginkgo-like forms are known {Ginkgophyllum, Baiera, 

 Saportaea, etc.). The araucariaceae are represented in the 

 Upper Carboniferous by Walchia and Schizodendron and in the 

 Permian by Walchia, Ulmannia and Gomphostrobus.^ The 

 cycadophytes appear to be represented as early as the Carbo- 

 niferous by fronds of Plagiozamites and Pterophyllum, and the 

 genus Sphenozamites is added in the Permian, but all of these 

 types are infrequent until the next era. 



2 The genus Voltzia of the Permian is often considered as representing the 

 Taxodieae. 



