ABSORPTION BY THE ROOTS OF PLANTS NOT OSMOTIC. 51 



hydrogen, as well as to present a suitable extent of surface 

 to the action of the light, but also to permit the Hberated 

 oxygen to escape as promptly as it becomes free. If we 

 compare in tliis respect the leaves of a wheat-plant witli 

 those of a turnip-plant, we find a striking diiTercnce in 

 their size, and hi the amount of water respectively con- 

 tained in them ; and a microscopic examination reveals 

 still greater differences. The wheat-plant has erect leaves, 

 which present to the Hght a much smaller surface than 

 the leaves of the tm-nip-plant, which overshadow the 

 ground, preventing the drying of the soil and the exha- 

 lation from it of carbonic acid. In the wheat-leaf the 

 stomates are equally thick on both sides ; in the turnip- 

 leaf they are much more numerous, although smaller than 

 in the wheat-leaf, and a far greater number of them are 

 found on the lower than on the upper side. 



All the facts known respecting the nutrition of plants 

 tend to prove that it is not by a mere osmotic process that 

 they absorb their food, but that the roots perform a very 

 definite active part in selecting from the amount of food 

 presented to them such matters and in sucli quantities as 

 are best suited to the plant. 



The influence of the roots is most manifest in the vege- 

 tation of marine and fresh-water plants, whose roots are 

 not in contact witli the soil. 



These plants receive their incombustible nutritive sub- 

 stances from a solution in which these elements are most 

 uniformly mixed and diffused ; and yet a comparative 

 analysis of the water and the ash-constituents of these 

 plants shows that each species absorbs from the same 

 solution different quantities of potash, lime, silicic acid, 

 and phosphoric acid. 



The ash of duckweed was found to contain 22 parts of 

 potash to 10 parts of chloride of sodium, whereas the 



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