i6 



THE ANATOMY OF WOODY PLANTS 



Having noted the varieties involved in wood structure in correla- 

 tion to the more variable conditions of environment present in 

 modern times, we may now with advantage direct our attention 

 to other features of organization which characterize the evolution 

 of the woody cylinder in the higher plants. Fig. 12 illustrates 

 the transverse section of the wood of a lepidodendrid, an ancient 



tree of the Paleozoic 

 age. It is clear in 

 this case that the 

 wood has a circular 

 outline correspond- 

 ing to that of the 

 stem as a whole. 

 Although there is no 

 indication of the ex- 

 istence of annual 

 incremen ts of 

 growth, the wood is 

 obviously divided 

 into a central mass, 

 in which the cells 

 are irregularly dis- 

 posed, surrounded 



, & zQne regu i ar iy 



^ m 



seriate and marked 



by the presence of 



wood rays. The portion of the wood which shows no linear disposi- 

 tion of its elements is known as the primary wood. This region of 

 the wood is sometimes designated the " cryptogamic " or "old" 

 wood, because it is particularly characteristic of the organization of 

 vascular cryptogams and of the older groups of plants generally. 

 As will be made clear later, the structure and mode of development 

 of wood of this category is of considerable importance from the 

 evolutionary point of view. The zone of secondary wood, outside 

 the primary or cryptogamic ligneous core, is conspicuous by reason 

 of its regular radial seriation and the presence of storage bands 



FIG. ii. Wood of a Paleozoic gymnosperm from 

 Prince Edward Island, Canada, showing absence of 

 annual rings and extremely simple organization. 



