THE RELATION OF PLANTS TO WATER 



141 



Palisadea cells ,Epidermi 



mini 

 cells 



latter fact may be practically demonstrated by the student in an 

 experiment similar to that illustrated in Fig. 67, b. If the stem 

 of a proper plant be cut off close to the root, as in the figure, 

 and a glass tube be fitted over the cut end of the stump by 

 means of rubber tubing, water will soon begin to well out of 

 the ducts, which have been 

 opened by cutting the stem. 

 This water, in an active 

 plant such as a coleus or 

 a begonia, will often rise 

 to a height of several feet 

 in a small glass tube, or to 

 that of from 40 to 50 feet in 

 the case of some trees. This 

 phenomenon (erroneously 

 called root pressure) is par- 

 tially explained by osmosis, 

 but the ultimate explanation 

 is as yet unknown. In nature 

 the outflow of water from 

 wounds usually occurs in 

 the spring before the leaves 

 unfold ; it ceases as soon as 

 the leaves expand and begin 

 . active transpiration. 



The soil salts taken into 

 the roots by osmosis move 

 with the water into the 

 ducts and up the stem to 

 the leaves, where, as we 

 have already noted, the salts are combined with the sugar, which 

 results from photosynthesis, to form the basic nitrogenous foods 

 for the entire plant. Nitrogenous foods are undoubtedly formed 

 also in other living cells of the plant body in the same manner 

 ^as in the leaves. 



Cytoplasmic sac 





Ducts 



aier ~\ ., > i 

 films &# Pa> 



FIG. 69. The path of water in the plant 



The lower portion of the figure shows the 

 structure of the soil and the relation of root 

 hairs to the soil particles and to the water 

 films; the upper portion illustrates the con- 

 nection of the leaf tissues with the ducts of 

 the stem 



