JRjSCOKD OF LIFE IN THE ROCKS. 



171 



medullary rays, and indicate plates of cellular or woody 

 substance. These make what is called the silver grain 

 of the wood. This division of plants includes the herbs, 

 shrubs, and timber trees, as they are seen grouped in 

 In this and Fig. 88 you see the differences be- 



Fig. 91. 





Fig. 91. 



tween the two classes of plants exhibited in the trunks 

 of two trees represented as cut square off. The same 

 can be said of the exogens as was said of the endogens 

 in relation to their zoological character. The remains of 



