74 ELEMENTARY BIOLOGY. [VIII, 



genous and mineral constituents derived from the roots are 

 combined with the carbon fixed 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 multi- 

 plying, and yet have no power of extracting 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 a material which contains nitrogen and hydrogen, 

 with oxygen and carbon, in some other shape than that of 

 carbonic anhydride. The analogy of Torula suggests a fluid 

 which contains in solution, either some ammoniacal salt com- 

 parable to ammonium tartrate, or a more complex compound 

 analogous to pepsin. Thus, the higher plant combines within 

 itself the two, physiologically 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 shew that the ascent of fluid from the root to 

 the leaves takes place, to a great extent, through the elon- 

 gated ducts of the wood, which not unfrequently open into 

 one another by their applied ends, and, in that way, form 

 very fine capillary tubes of considerable length. 



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 stomates. The push from below is the absorptive action 

 which takes place at the extremities of the rootlets, and 



