2i6 THE MOVEMENT OF WATER THROUGH THE PLANT 



the subject of much experimentation, and even more speculation. The en- 

 suing discussion of the "ascent of sap" through plants will be presented largely 

 in terms of its movement through trees, principally because much of the 

 experimental work on this problem has been performed on woody species. 

 Any explanation of this phenomenon which can be shown to be adequate for 

 tall trees should also prove satisfactory for vascular species of lesser stature. 

 The Path of Water Through the Plant. — Water enters the plant 

 through the epidermal cells and root hairs at or near the tips of the roots and 

 crosses the cortex, endodermis, and a part of the pericycle before it finally 



Fig. 52. Terminus of xylem elements in mesophyll. Redrawn from Eames and 



MacDaniels (1925). 



enters the lumina of the vessels or tracheids of the root xylem (Fig. 70). At 

 this point its upward movement begins. The xylem tissue is continuous from 

 just back of the root tips, through the roots, into and through the stems, the 

 petioles of leaves, and ultimately, usually only after much branching, termin- 

 ates in the mesophyll of the leaf (Fig. 52). The xylem tissue through which 

 the water moves is thus a continuous unit system within the body of the plant. 

 Along most of its course water moves en masse through the vessels or tracheids 

 of the xylem. At the termination of the xylem ducts in the leaves the water 

 passes into the adjacent mesophyll cells. In the mesophyll of the leaves it 

 moves from cell to cell, eventually most of it being lost from the cells by 



