THE EVOLUTION OF THE PLANT BODY 



41 



far more efficient protective tissue, against both water loss and 

 physical injury, than simple epidermal cells. Within the bark 

 and considered a portion of it, is the cortex, a continuation 

 of the cortex of roots. The cortex of the trunk occupies pro- 

 portionally much less space and is an inconspicuous tissue. Just 



Fig. 19. — A stem typically consists of an outer zone of bark and cortex 

 (dark area), a cylinder of wood cells marked by annual rings, and a central 

 pith of variable size. 



inside the cork and cortex tissues of the tree lies the inner bark 

 and sap wood, a zone of living cells which forms a ring around 

 the more central part of the trunk (fig. 20). This is the conductive 

 and growing part of the stem, continuous with the conductive 

 channels of the central cylinder of the root. The phloem tubes, 



