SPERMATOPHYTA 69 



the Silurian. Among living gymnosperms, the ginkgos are 

 their closest kin. 



The Cordaitales are confined to the Paleozoic and seem to have 

 furnished the predominant members of the gymnosperm forests 

 during the Devonian, and especially later during the Carbonif- 

 erous. 



Cordaites, named in honor of the early paleobotanist, Corda, 

 is the best known genus. In the Upper Devonian black shale 

 of Indiana are trunks two feet in diameter and tw^enty feet 

 long, indicating trees probably one hundred to one hundred 

 and fifty feet high (Fig. 23). In Pennsylvanian shales in 

 New Brunswick the leaves are packed in layers like those of 

 modern forests. 



1. Describe the appearance of Cordaites. 



2. How were casts of its pith cavity formed ? What are they 

 called ? 



3. Discuss its relationships and probable evolution. 



4. Describe a forest of Carboniferous time as you think it 

 must have appeared. 



Order D, Ginkgoales 



An isolated order of gymnosperms, represented at present 

 by only one genus and one species. Ginkgo biloba, — the maiden- 

 hair tree, but with the long and varied ancient history that 

 such isolation in the existing flora ahvays indicates. The ginkgo 

 is a tree which in general appearance and in the anatomy of the 

 stem closely resembles the conifers. The leaves, however, are 

 fan-Uke in outline like the large leaflets of the maidenhair fern, 

 and are shed each year. 



A primitive character which it possesses along with the cycads, 

 and which is reminiscent of a fern ancestry, is the motility 

 of the male cells in fertilization, already mentioned (p. 66). 



The ginkgo tree has long been cultivated by the Chinese and 

 Japanese, especially in the temple grounds ; now a common 

 cultivated tree, it is not surely known wild in any part of the 



