PLANT FOOD 



139 



weight. Other substances may be present in varying 

 proportions, but the two groups named above are found 

 in all plants without exception, and so we may conclude 

 that (with the possible addition of chlorine) they form the 

 indispensable elements of plant food. Carbon, hydrogen, 

 oxygen, nitrogen, sulphur, and phosphorus compose the 

 structure of which the plant is built. The other four do 

 not enter into the substance as component parts, but aid 

 'in the chemical processes by which the life functions of 

 the plant are carried on, and are none the less essential 

 elements of its food. Figure 283 shows the difference 

 between a plant grown in a solution where all the food 

 elements are present and others in which some of them 

 are lacking. 



198. How Plants obtain their Food. — With a few doubt- 

 ful exceptions, plants cannot assimilate their food unless 

 it is in a liquid or gaseous form. Of the gases, carbon 

 dioxide, oxygen, and hydrogen can be freely absorbed from 

 the air or from water 

 with various substances 

 in solution, but most 

 plants are so constituted 

 that they cannot absorb 

 free nitrogen from the 

 air ; they can take it only 

 in the form of compounds 

 from nitrates dissolved 

 in the soil, and hence the 

 importance of ammonia 

 and other nitrogenous 

 compounds in artificial fertilizers. Some of the pea family, 

 however, bear on their roots little tubers containing minute 

 organisms called bacteria, which have the power of ex- 

 tracting nitrogen directly from the free air mingled with 

 the soil ; and hence, wherever these tuber-bearing legumes 

 are present the soil is found to be enriched with nitrogen 

 in a form ready for use. 



284. — Roots of soy bean bearing tubercle- 

 forming bacteria. 



