60 



PRACTICAL COURSE IN BOTANY 



tion, 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 ingredients 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 74 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. 



63. How plants obtain their food material. Plants 

 obtain their supply of the various mineral salts from solu- 

 tions in the soil water which 

 they absorb through their 

 roots. With a few doubtful 

 exceptions, they cannot as- 

 similate 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 va- 

 rious substances in solution, 

 but most plants are so con- 

 stituted 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 formed by minute organisms called bacteria, which 

 have the power of extracting nitrogen directly from the 

 free air mingled with the soil ; and hence the soil in which 

 these tuber-bearing legumes decay is enriched with nitrogen 

 in a form ready for use. 



FIG. 75. Roots of soy bean bearing 

 tubercle-forming bacteria. 



