34 COMPOUND ORGANS OF PLANTS. 



In growing, the roots of plants therefore do not elongate 

 through their entire length, but increase by the addition of 

 matter to their advancing points, very much like an icicle, 

 except that the new matter is added from within and not from 

 without. Growing in a medium of such unequal resistance 

 as the soil, they elongated through their entire length they 

 would, when they encountered any obstacle, be thrown into 

 knotted and contorted forms, which would prevent their acting 

 as conduits of food from the soil, which is their peculiar office. 

 But as they only elongate by the formation of fresh tissue at 

 their extremities, they are thus enabled to accommodate them- 

 selves to the nature of the soil in which they grow ; and, 

 should any thing impede their progress, they sustain no injury, 

 but following the surface of the opposing matter, they grow 

 and extend themselves until they again enter a softer and 

 more favorable medium. In this manner they penetrate the 

 soil, as it were, in search of food, insinuating themselves into 

 the minutest crevices of rocks, and extending themselves from 

 place to place, as the nutriment in their own immediate 

 neighborhood is consumed. 



Now all newly formed vegetable is extremely hygrometrioal, 

 and hence absorption takes place throughout the whole extent 

 of the newly formed tissue. 



The law which regulates this absorption has been recently 

 discovered by M. Dutroohet, a distinguished French physiolo- 

 gist. It is this : if two fluids of unequal densities be separated 

 by an animal or vegetable membrane, the denser fluid will 

 draw the lighter through the membrane with a force propor- 

 tional to the difi'erence of density of the two fluids. A simple 

 experiment will illustrate this. (Pig- 9). 



Take a short tube, and cover one end with a piece of blad- 



