i68 



THE ANATOMY OF WOODY PLANTS 



found without exception in the roots of all vascular plants. The 

 situation diagrammatically delineated in b prevails in the stem 

 organs of the ferns and their allies, the lower gymnosperms. In 

 the higher gymnosperms and the angiosperms the axial organs 

 exemplify almost exclusively, in living types at any rate,' the 

 endarch condition diagrammatically represented in c. As might be 

 expected, the primitive occurrence of exarch and mesarch conditions 



B 



FIG. 121. Diagram to indicate the relations of protoxylem to metaxylem in 

 vascular plants. Explanation in the text. 



in the case of the stem can frequently be inferred from the anatom- 

 ical situation presented by the leaves. The universal presence of 

 the exarch type of primary wood in the root furnishes good evidence 

 that this was the original type of organization and development 

 of the primary wood in the vascular series. 



It has been made evident in an earlier chapter that, in addition 

 to the primary organization of the wood consisting of tracheary and 

 parenchymatous elements arranged in irregular order, there is 

 often present, particularly in arboreal types and perennials, ancient 



