494 ORGANOLOGY. 



stand constantly in need of the inorganic elements of the earth, but only 

 in very small quantity. They need, for example, carbonate of potassa ; 

 but a considerable quantity of this salt is either dissolved out of the soil 

 by rain, or by a constant evaporation the water is so concentrated 

 that it exerts a destructive influence on plants. Hence there exists a 

 necessity either that the salt must belong to those difficult of solution, 

 as gypsum, carbonate of lime, &c., in which case all the water taken up 

 would contain the same percentage of salts, because the diminution of 

 the quantity of water is attended with a separation of a certain quantity 

 of the insoluble salt ; or else the salts must be delivered, as are the 

 phosphates and carbonates of the alkalies, very gradually by the action 

 of the atmosphere, and subsequently decomposition. This last is the 

 case with silica, which is so difficult of solution, and so quickly resumes 

 its original solid condition. The plants which need this ingredient can 

 only obtain it when it exists in the form of the silicates, or when, by the 

 decomposition of organic substances in the earth, the requisite quantity 

 is set free. 



Gases enter into the same category as fluid matters, since all gases 

 that come under our consideration here are more or less soluble in water. 

 The relation also of gases separated from each other by a moist homo- 

 geneous membrane is the same, whether they are insoluble in water or 

 not. 



2. Of the Form of the Processes. 



195. We must regard the processes of absorption and excretion 

 from three points of view, according to the form of the matter. 



1. The absorption and excretion of fluid matter, which embraces 

 the question of the interchange of the matters, and of independent 

 excretion. 



2. The absorption and excretion of vapour, which always occurs 

 independently. 



3. The absorption and excretion of gases, which is carried on 

 by interchange, and also independently. 



These three processes may be called nutrition, perspiration, and 

 respiration, provided we do not suppose them analogous to the like- 

 named processes in animals. 



196. The absorption of fluid matters occurs probably mostly, 

 if not always, in connection with a simultaneous excretion of smaller 

 amount, according to the laws of endosmose. 



In reference to endosmose, there are three relations of the plant 

 to the media in which it vegetates to be distinguished. The sim- 

 plest and most natural case is the vegetation of plants in water, or 

 in a soil perfectly saturated with water (as in bogs). In this case, 

 the cell-walls are in immediate contact with the fluid, and receive 

 it by endosmose, so long as no covering prevents it. A trifling 

 difference between the chemical or physical contents of the cells 

 and the surrounding water is sufficient to sustain the endosmotic 

 process. 



The second case is that in Avhich the cells come in contact with 

 solid matter, endowed with the property of absorbing water. In 



