368 GROWTH OF THE PLANT. 



the cotyledons, which, as they rise above the earth, 

 acquire a green colour. Here the first stage of vege- 

 table life ends, the chemically exciting process is at an 

 end, and a new stimulus is required to continue in full 

 activity the vital powers. Carbonic acid is no longer 

 given off. 



The cotyledons, which are two opposite roundish 

 leaves, act as the lungs ; by them carbonic acid taken 

 from the atmosphere is absorbed and carried by a circu- 

 lating process, now in full activity, through the young 

 plant. The carbonic acid, a compound of carbon and 

 oxygen, is decomposed; it is deprived of its carbon, which 

 is retained by the plant, and oxygen is exhaled. The 

 plant at this period is little more than an arrangement 

 of cellular tissue, a very slight development of vascular 

 and fibrous tissue appearing as a cylinder lying in the 

 centre of the sheath. At this point, however, we begin 

 more distinctly to trace the operations of the new 

 power ; the impulses of life are strikingly evident. 



The young root is now lengthening, and absorbing 

 from the moisture in the soil, which always contains 

 some soluble salts, a portion of its nutriment, which is 

 impelled upwards by a force probably capillary attrac- 

 tion and endosmose action combined to the point from 

 which the plumule springs. Capillary force raises the 

 fluids through the tubes in the stalk, and conveys 

 them to the veins in the leaves, while the endosmose 

 force diffuses them through the vegetable tissues. The 

 plumule first ascends as a little twig, and, at the same time, 

 by exerting a more energetic action on the carbonic 

 acid than the cotyledons have done, the carbon retained 

 by them being only so much as is necessary to form 

 chlorophylle, or the green colouring matter of leaves, 

 some wood is deposited in the centre of the radicle. 

 From this time the process of lignification goes on 

 through all the fabric, the increase, and indeed the 



