468 THE BEAN PLANT. [CHAP. 



from the roots, where they are absorbed, through the stem 

 to the leaves. And, at whatever parts of the plant the nitro- 

 genous and mineral constituents derived from the roots 

 are combined with the carbohydrates produced in the leaves, 

 the resulting compound must be diffused thence, in order to 

 reach the deep-seated cells, such for instance as those of 

 the cambium layer and those of the roots, which are 

 growing and multiplying, and yet have no power of ex- 

 tracting carbon directly from carbonic anhydride. In fact, 

 those cells which contain no chlorophyll, and are out of 

 the reach of light, must live after the fashion of Torula ; 

 and manufacture their protein out of the nitrates and salts 

 of ammonia taken up by the root, in combination with such 

 bodies as grape-sugar, already formed in the leaves. Thus, 

 the higher plant combines within itself the two, physiologi- 

 cally distinct, lower types of the Fungus and the Alga. 



That some sort of circulation of fluids must take place 

 in the body of a plant, therefore, appears to be certain, but 

 the details of the process are by no means clear. There is 

 evidence to show that the ascent of fluid from the root to 

 the leaves takes place, to a great extent, through the vessels 

 of the wood, which in the higher plants have their transverse 

 walls broken down so as to form very fine capillary tubes 

 traversing both stem and root. 



The mechanism by which this ascent is effected is of two 

 kinds ; there is a pull from above, and there is a push from 

 below. The pull from above is the evaporation which takes . 

 place at the surface of the plant, and especially in the air- 

 passages of the leaves, where the thin-walled cells of the 

 parenchyma are surrounded, on almost all sides, with air, 

 which communicates directly with the atmosphere through 

 the stomata. The push from below is due in the first 

 instance to the absorptive action of the root hairs. The 



