32 THE PHYSIOLOGY OF PLAIs^TS 



the plant as a source of this element. The atmosphere, how- 

 ever, M'hich surrounds the plant affords it all the carbon 

 which it requires. The leaves absorb carbon from the air 

 in the form of carbonic acid, and decompose the latter in 

 their cells under the influence of light, giving off the surplus 

 oxygen. 



Nitrogen is the substance which is chiefly concerned in the 

 building up of young tissues, but it is also present throughout 

 the whole life of the cell in the substance which we termed 

 protoplasm. 



Alkaloids, which are present in many plants, and asparagiu, 

 which is concerned in the regeneration of the albuminous sub- 

 stances, are formed from the protoplasm, and contain nitrogen. 

 Most of our cultivated plants have to take their nitrogen from 

 the soil ; it is only the leguminous plants which are able to 

 take up the nitrogen through their leaves from the atmosphere, 

 of which nitrogen is the chief constituent. As far as our 

 present scientific knowledge goes, the leguminous plants can 

 subsist on the nitrogen which they take out of the air ; while 

 cereals, fruit-trees, and indeed all other Phanerogams, must 

 obtain this substance in some soluble form from the soil. 

 Nitrates seem to be the most suitable form of salt from which 

 plants obtain their nitrogen. Ammonia, which can probably 

 be absorbed in minute quantities even in a gaseous state, is 

 less suitable. 



Sulphur and phosphorus are of some importance in the for- 

 mation of albuminous substances ; both are taken entirely from 

 the soil, where they are found in the form of calcium salts. 

 This is especially the case with sulphur, which is widely dis- 

 tributed as calcium sulphate (gypsum), while calcium phos- 

 phate is of less frequent occurrence, and being only very 

 slightly soluble, it is advantageously replaced by a potassium 

 salt. In some places phosphoric acid is found very largely in 

 the soil in combination with iron. Plants which are deficient 

 in phosphorus generally assume a red coloration. 



Potassium, which is chiefly found in very young tissues, and 

 disappears in the older ones, is probably connected with the 

 formation of the earbo-hydrates, i.e., with those organic sub- 

 stances which consist of carbon, hydrogen, and oxygen, and in 



