76 ELEMENTARY BIOLOGY. [CHAP. 



leaves. And it can be proved, experimentally, that fresh 

 green leaves possess this power to a remarkable extent. 



On the other hand, it is clear that, when a plant is grown 

 under the conditions described, the nitrogenous and mineral 

 constituents of its food can reach the leaves only by passing 

 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 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 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 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 solu- 

 tion, either some ammoniacal salt comparable 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. 



