THE WATER-RELATION 



99 



This brings us to the first section of the route which most of the 

 water absorbed by the root-hairs pursues through the plant, in the 

 so-called Transpiration Stream. The water passes laterally from 

 the root-hairs across the cortex of 

 the root, through the endodermis 

 and pericycle and into the xylem 

 vessels (Fig. 68). Until the xylem 

 is reached the water passes through 

 living cells, each cell forming the 

 usual osmotic unit. As in the case 

 of the root-hairs, so with these 

 inner cells, if there were not a 

 constant movement of water from 

 the innermost living cells into the 

 xylem vessels, all the living cells 

 would soon become fully turgid, 

 and a static condition would be 

 reached. The fact that a constant 

 movement of the type mentioned 

 does exist results in the mainten- 

 ance of an absorptive capacity in 

 those inner living cells. These 

 in consequence take up water 

 osmotically from the cells lying 

 next outside them, a process which 

 extends outwards until the root- 

 hairs and soil are reached. A 

 lateral flow of water from the soil 

 through the living cells of the 

 root is thus maintained till it f rrows, : ll Yh l root " hairs . we . re proportionaii : 



longer than they appear in the drawing. The 



reaches the Xvlem. The move- cell-contents are shown in some cases, dia- 



j ' U,V/ grammatically. r.h. = root-hair ; c= cortex ; 



ment of water from the inner- «= endodermis ; *y= xylem . (*i 33 .) (With 



acknowledgments to Pmestley.) 



most living cells into the xylem 



will be reserved for later consideration. At this point we leave it 

 in its journey through the plant, and go on to study the course of 

 events above soil-level. 



Fig. 68. 



Part of a transverse section of a Pea root to 

 show the direction of movement of water (see 



Transpiration. 



We have seen in previous chapters how the leaves and young 

 stems of the plant are covered by a protective cuticle, which is highly 



