102 BIOLOGY. [Book ir. 



In the complex plant, the air is therefore subjected to a sort 

 of circulation, which Dutrochet justly compares with the aerian 

 circulation in the trachese of insects which are likewise 

 furnished with stomata. Once introduced into the vegetal 

 tissues, the air is absorbed by the anatomical elements,' and by 

 the sap or the blastemas which it contributes to elaborate. 

 Finally it is probable that a part of the oxygen resulting from 

 the reduction of the carbonic acid by the green parts, and which 

 is consequently in the nascent state, that is to say, eager to 

 enter into new combinations, is also forthwith absorbed, or even 

 mechanically driven into the canals and cellular interstices. 



4. Sap Elahorated or Descendent. 



The water of the soil, more or less charged with the sub- 

 stances in dissolution, penetrates by endosmosis into the cells of 

 the spongioles, and thence is impelled into the vessels, the 

 meatus, the anatomical elements, step by step. Finally it is in 

 some sort inhaled by the whole of the tissues, which all have 

 need of nourishment, and more specially by the leaves and the 

 green parts. These chlorophyllian tissues are the special 

 exhaling and assimilative organs, in the midst whereof the sap 

 undergoes a very important elaboration, and in a fashion passes 

 into the state of true blastema. So far in effect the sap was 

 nothing more than a simple mineral solution. In the leaves the 

 sap is vitalised, it becomes an organisable liquid. It is only 

 after this metamorphosis of the sap that new anatomical elements 

 can be formed at its expense, that the organism expands and 

 grows. This sap thus modified has been called sap descendent, 

 because its ordinary course is from the leaves to the roots. It is 

 interesting to follow the flow of this elaborated sap, to see how 

 it passes and distributes itself from the leaves to the root of the 

 plant. 



, It is needful for us here to take anew into account the effect 

 of the endosmosis, which plays moreover such a foremost part in 



