ASSIMILATION OF NITROGEN BY PLANTS 223 



been transformed into inorganic form by the activity of soil 

 bacteria. Similarly, the carbon present in humus cannot be 

 used 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 microorganisms. Thus it is seen 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 nutri- 

 tion. 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 

 nongreen plants like the fungi and certain higher plants of 

 parasitic nature, such as Orohanche. 



The salts of ammonium and those of nitric acid are the inor- 

 ganic compounds of nitrogen commonly found in soil. In 

 water cultures, the salts of nitric acid produce the better growth. 

 Consequently, it was believed for a long time that nitrogen was 

 assimilated by the plant only in this form. In cases where 

 fertilization with ammonium salts (a sulphate is most frequently 

 used for this purpose) produced fairly good results in soil cultures, 

 this was attributed to the fact that ammonia is first oxidized by 

 bacteria into nitric acid, only then becoming available to the 

 plant. 



Experiments carried out with sterile cultures have shown this 

 concept to be false. The root system of plants can absorb 

 ammonium salts directly, and provided that they are in a suffi- 

 ciently dilute solution, they cannot be considered inferior to 

 nitrates as a source of nitrogen for plants. In higher concen- 

 trations, however, ammonium salts are toxic. They cause a 

 weaker and more irregular growth of the roots and hence have a 

 harmful effect on the general development of a plant. 



As a general rule, it may be said that, under conditions favor- 

 able for the growth of plants, ammonium salts are assimilated 

 just as well as nitrates. But as soon as external factors, such as 

 temperature, illumination, and soil acidity, do not quite corre- 

 spond to the requirements of the plant, the toxic properties of 



