Il8 GROWTH AND WORK OF PLANTS 



198. Sources of plant food. The different elements, which 

 have been found necessary constituents of plant food, are not 

 taken up by the plant as elements, except in rare cases, and possi- 

 bly also with the exception of the oxygen of respiration. Oxygen 

 is taken into the plant as an element in the process of respiration. 

 If this oxygen merely assisted in the combustion of plant material 

 it would not in this instance be a food constituent, but there are 

 reasons for believing that it is first assimilated into the living matter 

 (see paragraph 185). Nitrogen is taken up as an element in the 

 nutrition of a few specialized bacteria (see fixation of nitrogen, 

 paragraphs 201-203). The mineral substances found in the ash 

 of plants when they are burned, containing among other things 

 calcium, magnesium, phosphorus, sulphur, iron, the plant takes 

 up from the soil in the form of nitrates, sulphates, phosphates,* 

 etc., which are formed during the weathering and disintegration 

 of rocks. | The water (H 2 O) of the plant is absorbed from the 

 soil. This furnishes part of the hydrogen and oxygen. The 

 source of the carbon for green plants is the carbon dioxide (CO 2 ) 

 of the air, which the plant uses during photosynthesis. Nitrogen 

 is obtained by absorption of nitrates from the soil and from some 

 other compounds of nitrogen. For example, in cultivated or 

 waste fields, the nitrogen food is mostly in the form of nitrates, 

 while nitrates are scarce in the forest where there is an abundance 

 of decaying leaves and humus. In the forest the nitrogen food 

 is chiefly in the form of ammonia compounds (NH 3 ). 



199. Most of the substances used as plant food from 

 the soil exist there in quantities sufficient to last plants 

 for many years. But nitrogen compounds are rare,J and if 



* Examples, potassium nitrate, calcium sulphate, magnesium sulphate, 

 calcium phosphate, etc. 



f The small particles of rock make the basis of the soil, while dead plant 

 remains furnish the organic matter and humus which give it a darker color 

 and make it more retentive of moisture. 



J Phosphates are also comparatively rare in many soils, and during 

 continued cropping when they are not returned to the soil, the soils become 

 deficient. Phosphorus occurs in the oldest rocks and these phosphates 

 appear in the soil from disintegration of rock. By the growth of plants they 



