ABSORPTION OF NITROGEN FROM SOIL AND AIR 53 



f^ 



special devices, while the seed is thoroughly washed with disin- 

 fectants previous to germination, and then germination is allowed 

 to proceed in a sterile medium. As a result, the roots of the plant 

 develop beneath the cotton with a complete absence of bacteria, 

 while its leaves spread freely in the air (Fig. 22). 



Experiments with sterile cultures have definitely shown that 

 many of the organic compounds of nitrogen such as amino acids, 

 lecithin, etc., may be assimilated by plants. Their assimilation, 

 however, proceeds at a very 

 slow rate. Plants developing 

 in such media are therefore 

 considerably checked in growth 

 as compared with plants re- 

 ceiving nitrates or ammonium 

 salts. Hence, it may be con- 

 cluded that mineral com- 

 pounds of nitrogen are better 

 sources of this element for 

 plants than the organic forms, 

 and that nitrogen in humus is 

 utilized only after it has been 

 transformed into inorganic 

 form by the activity of soil 

 bacteria. Similarly, the carbon 

 present in humus cannot be 

 utilized by plants until it is 

 changed into the form of 

 gaseous carbon dioxide, which 

 is likewise produced in soils as a result of the life activity of micro- 

 organisms. Thus it is observed that a very close connection exists 

 between the nutrition of green plants and the work of soil organisms. 



The ability to construct its body from simple inorganic sub- 

 stances marks a distinction between a green plant and an animal, 

 the latter absolutely requiring organic compounds for its nutrition. 

 Green plants, therefore, may be called autotrophic organisms. 

 They have the capacity of independent nutrition, as contrasted 

 with heterotrophic organisms, which are nourished at the expense 

 of organic substances produced by other living things. In the 

 latter group are included animals as well as non-green plants, such 

 as the fungi and certain higher plants of parasitic nature. 



Fig. 



22. — Sterile water culture of a corn 

 plant (redrawn after Knudson). 



