90 BIOLOGY. [BOOK n. 



1. Formation and Circulation of the Sap. 



Let us take as type a complete plant, a dicotyledon, plung- 

 ing its roots into the ground, displaying its branches in the air. 

 In the spring such a plant impregnates itself incessantly with 

 the materials which it appropriates from the exterior medium. 

 It absorbs them by the roots, by the leaves, by the bark. It is 

 by the osmotic process that the roots draw from the soil the 

 first materials of the sap. It is the delicate cells of the 

 extremities of the roots, radicellular spongioles, which are 

 the principal agents of absorption. These cells contain a pro- 

 toplasm dense and albuminous, coagulable by nitric acid ; 

 they are in contact with the soil by means of their cellular 

 membrane, by means of the hairs which garnish them ; they are 

 thus in conditions very favourable to osmotic absorption. It is, 

 moreover, easy to prove that is owing to the process entirely 

 physical of endosmosis, that the roots saturate themselves with 

 the humidity of the soil, for all that is needful to arrest their 

 work of absorption is to plunge them in a saccharine and dense 

 solution. In the soil, on the contrary, the water, relatively little 

 charged with dissolved substances, moistens the extremities of 

 the roots, penetrates by endosmosis into their anatomical ele- 

 ments, and mingles there with their protoplasm, bringing with it 

 ammoniac salts, phosphates, salts of potash, and so on. 



But to accomplish their office the hairs of the roots need, like 

 all organised cells, nourishment, that is to say, need to be 

 oxydised, to absorb oxygen, and to exhale carbonic acid. Con- 

 sequently, the penetration of the air into the soil into which 

 the roots plunge is indispensable to the maintenance of the life 

 of plants. The exhalation of the carbonic acid by the roots has 

 also its utility. In effect it is from the presence of this carbonic 

 acid that certain salts become soluble in water and can thus 

 penetrate into the radical cells. The case is the same, for 

 example, with certain phosphates, and no doubt also for silica, 

 and so on. Once introduced into the cells of the spongioles, of 



