32 STRUCTURAL AND PHYSIOLOGICAL BOTANY. 



248. The perfect developement of a plant is therefore de- 

 pendent on the presence of alkalies or alkaline matter ; for 

 when these substances are totally wanting, growth will be 

 arrested ; and when they are deficient, it must be impeded in 

 proportion. 



249. But other substances besides alkalies are required to 

 sustain the life of plants. Phosphoric acid has been found in 

 the ashes of all plants hitherto examined ; and common salt, 

 sulphate of potash, nitre, salts of iron and copper, chloride of 

 potassium, and other matters, may be regarded as necessary 

 constituents of several plants. 



250. Therefore it is indispensable that every plant should 

 find in the soil it is cultivated in those inorganic constituents 

 which nature has rendered necessary to it, just as it is neces- 

 sary for animals that they should find in their food the phos- 

 phates of lime and magnesia, which harden their bones. 



251. As soon as food is absorbed, it begins to ascend into 

 the stem, or to diffuse itself through the system, and receives 

 the name of sap. 



252. In the course of the sap upwards, the water and car- 

 bonic acid are partially decomposed and their elements are 

 deposited along with nitrogen in the interior of the tissue, 

 forming a layer over the interior of every cell and vessel, 

 which thus become in part solidified. 



253. As soon as the sap reaches the leaves or the surface of 

 the bark, green matter, or occasionally some other colour, is 

 formed, provided the part is exposed to light. 



254. This appears to arise chiefly from the decomposition of 

 carbonic acid, ammonia, and water, when the carbon, nitro- 

 gen, and hydrogen are fixed by the plant, and the oxygen 

 restored to the atmosphere. Such action is called the assimi- 

 lating power of plants. 



Plants are, therefore, the great purifiers of the atmosphere, consuming the pro- 

 ducts of animal respiration and of all organic putrefaction, and converting them 

 again into matter suited to the wants of man. 



255. In the absence of light, plants re-absorb oxygen from 

 the atmosphere, and re-combine it with the matter they con- 

 tain, to be again liberated at the return of light. 



256. They also, at all times, especially at night, part with 

 carbonic acid in small quantities. 



