108 THE NATURE AND WORK OF PLANTS 



and smaller quantities at other times. There is 

 still much to be found out about the ascent of 

 sap. 



145. Growth of stems. — The manner in which 

 stems grow and increase in size bears a close rela- 

 tion to the arrangement of the mechanical elements. 

 The arrangement of the living tissues is different 

 in the various types, of course. From the many 

 forms of the stem it will be most profitable to select 

 a tree and the cornstalk for study of this feature. 



146. Action of einhnjonk tissue of a tree. — The 

 living cells of a tree which constantly divide, form- 

 ing others which pass into dead tissue, lie immedi- 

 ately underneath the bark and completely sheathe 

 the trunk, forming a small cone of delicate cells at 

 the tips of the stems and branches and in the buds. 

 Dead cells are constantly being formed on the inner 

 side of this layer and added to the wood. The 

 cells formed in any one season are distinguishable 

 from those of the last season and constitute an 

 annual ring, though sometimes two rings may be 

 formed in one summer. At the same time dead 

 cells are being added to the bark on the outer 

 side of the living tissue. The living- cells at the 



