94 



BIOLOGY OF PLANTS. 



plants. It thus presents these matters in a form capable of 

 entering the organs of absorption. 



(3) In a similar manner, also, by passing over animal mat- 

 ters, such as horns, bones, wool and animal manures, water 

 dissolves those parts which are fitted for nutriment, as fast as 

 formed, and, in a similar way, facilitates their introduction 

 into the vegetable organs. 



(4) Water also absorbs various gaseous compounds, such 

 as common air, carbonic acid and ammonia ; prevents them 

 from escaping beyond the reach of the plant, and conveys 

 them into the appropriate vessels. The quantity of carbonic 

 acid which water is capable of absorbing is equal, at the com- 

 mon temperature and pressure, to its own volume. 



The quantity of ammonia is much larger, amounting, ac- 

 cording to Thompson, in some cases, to seven hundred and 

 eighty times its bulk. These substances are brought to the 

 earth by every shower of rain, or are absorbed as soon as 

 formed, in the soil. The agency of water in this respect is 

 of the highest importance in the nourishment of plants, because 

 most of the manures which are added to soils, throw off in the 

 process of decay, large quantities of these gases, which would 

 be mostly lost for the purposes of nutrition, were they not in- 

 stantly absorbed by the water, and retained for the use of the 

 plant. The air which the water holds in solution, is much less, 

 and, as we have seen, is needed in the process of germination, 

 in a free state. Generally, aJl bodies which in any way con- 

 stitute the food of plants, must first be dissolved in water be- 

 fore they can be introduced into the roots, and become a part 

 of the living system. 



2. Chemical agency of ivattr. In the process of decay, 

 putrefaction, fermentation, etc. by which food is prepared for 

 the nourishment of plants, water is always decomposed, its 

 hydrogen combining with the oxygen of the air, and its oxy- 

 gen with the carbon of the decaying vegetable, forming car- 

 bonic acid, and other compounds, which are found in the hu- 



