THE LYCOPODIALES 263 



such as Lycopodium or Selaginella. On the contrary, the most 

 valuable results from the standpoint of the doctrine of descent 

 can be derived from the study of the complicated arboreal extinct 

 types known as lepidodendrids and sigillarians. It has been made 

 clear in the preceding paragraphs that study of the last-mentioned 

 types throws extremely important light on the origin of storage 

 devices in the primary and secondary wood and makes clear the 

 status of the so-called medullary rays. The Lycopodiales, although 

 largely extinct, cannot accordingly be neglected by students of 

 the data of evolution, and they supply valuable evidence, if any 

 were needed, for the necessity of a knowledge of extinct forms 

 as an indispensable basis for the understanding of organisms now 

 living. 



