ABSORPTION BY THE BOOTS OF PLANTS NOT OSMOTIC. 63 



whereas in the plant they contain no nitrogen. All 

 processes of vegetative life tend simply to produce the 

 elements of the seed. The plant only lives in generat- 

 ing the egg-constituents and the egg itself ; the animal 

 only lives by destroying these very egg-constituents. 



On one and the same soil equally suited for the tur- 

 nip and the wheat-plant, the former produces for the 

 same amount of azotised substance twice as much un- 

 azotised matter as the latter. It is manifest that if two 

 plants produce within the same time different quanti- 

 ties of hydrates of carbon (wood, sugar, and amylum), 

 the organs of decomposition must be arranged in a man- 

 ner not only to afford adequate room for the carbonic 

 acid supplying the carbon, and for the water supplying 

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

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

 liberated oxygen to escape as promptly as it becomes 

 free. If we compare in this respect the leaves of a 

 wheat-plant with those of a turnip-plant, we find a 

 striking difference in their size, and in the amount of 

 water respectively contained in them ; and a microscopic 

 examination reveals still greater differences. The wheat- 

 plant has erect leaves, which present to the light a 

 much smaller surface than the leaves of the turnip- 

 plant, which overshadow the ground, preventing the 

 drying of the soil and the exhalation 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 

 such quantities as are best suited to the plant. 



The influence of the roots is most manifest in the 

 vegetation of marine and fresh-water plants, whose roots 

 are not in contact with the soil. 



