CHAPTER XXI 

 THE ASCENT OF SAP 



The ascent from earth to heaven is not easy. 



— Seneca. 



Of the many problems which have long baffled physiologists, 

 that of the ascent of sap has been one of the most perplexing. 

 Everyone is familiar with the fact that water does rise from the 

 soil to the tops of the highest trees such as the giant redwoods, 

 which are over 300 feet tall, or to the tops of the eucalyptus trees 

 of Australia, which are even taller, reaching a height of 400 feet 

 and over in exceptional cases. How is the water raised to such great 

 altitudes? What are the forces which are at work to lift such 

 large masses of water such great distances? The problem is very 

 complex because of the variety of forces which come into play, 

 but the chief obstacle to its solution has been our ignorance of 

 the physical and chemical principles involved rather than any 

 inherent mystery in the problem. 



Path of Water.— The path of water is from the root to the leaf 

 through the wood or xylem. Water enters the root through the 

 root hairs and traverses the cortex to the central cylinder, where 

 it passes into the vessels and tracheids, going up through them to 

 the leaves. Here the water goes from the vessels and enters the 

 living cells which surround them, where it passes from cell to 

 cell until it is evaporated from a mesophyll cell abutting on an 

 air space. To be sure, some water passes off from the cuticle, and 

 some is used up in the processes which go on within the plant as 

 outlined at the beginning of the last chapter, but the path sketched 

 is that followed by most of the water which enters the plant. 



The walls of the endodermis, particularly the radial walls, are 

 partially cutinized, while the tangential walls of the endodermal 

 cells near the end of the xylem regions of the root remain thin as 

 long as the root hairs in that portion of the root are functioning. 

 These passage cells thus permit the water which comes in from the 

 cortex to enter the xylem without passing through the phloem 

 of the root; and the sieve tubes of the phloem are thus kept from 



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