236 PLANT SOCIOLOGY 



Phoma, Gymnoascus, Alternaria, etc.) and algae. All of these seem 

 able to fix free nitrogen. Some such fungi and bacteria seem to stand 

 in a symbiotic relation to each other. 



The tubercle bacteria {Bacterium radicicola, B. heijerinckii) proba- 

 bly fix nitrogen only in symbiosis with higher plants (mostly 

 Leguminosae but also Podocarpus, Alnus, Elaeagnus, and Hippophae). 

 The role they play in agriculture has been well known since the days 

 of the classical investigations of Hellriegel and Wilfahrt, But we 

 know nothing about their significance in the development of natural 

 vegetation. It is probable that the tubercle bacteria make possible, 

 or at least greatly favor, the usually luxuriant growth of numerous 

 Leguminosae (Lupinus, Lotus, Astragalus, Ornithopus, etc.) upon the 

 sandy soils of southern Europe and northern Africa. 



The gain in nitrogen content of untilled, natural soils must be very 

 considerable. Two fields in central England, which had been given 

 no nitrogen fertilization for 22 and 24 years, showed, according to 

 Russell (1927), a total increase in nitrogen content of 2,162 and 1,567 

 kg. per hectare. These gains of nitrogen were proportionate to the 

 abundance of Leguminosae. 



Microorganisms not only contribute to the increase of the food 

 resources of the soil but also release inorganic food materials. Thus, 

 by their mediation, the cycle which transforms dead organic waste 

 into assimilable plant food is completed. A whole series of ecologically 

 important microorganisms and groups of organisms (mostly bacteria) 

 may be recognized. Physiologically, some have very definite, highly 

 specialized activities; others are indifferent, adaptable to many 

 different conditions of life. But their direct or indirect relation to 

 plant sociology has received little investigation. 



Ammonium Formation (Ammonification). — Nitrates are the most 

 effective nitrogen compounds for the nourishment of plants. For 

 all higher plants not living in symbiosis with the tubercle bacteria, 

 nitric acid forms, if not the only,^ at least by far the most important 

 nitrogenous food substance. The nitrogen compounds present in the 

 soil must generally be converted into nitric acid, HNO3, or its salts in 

 order to fulfill their physiological function. But this transformation 

 can take place only with the cooperation of nitrifying bacteria. It 

 proceeds step by step. The undecomposed organic matter first goes 



^ Cf. Schreiner, A., The organic constituents of soil, Science 36, 1912. According 

 to the investigations by Ziegenspcck (1922), mycotrophic plants do not assimilate 

 nitrogen in the form of nitrates but in that of ammonium compovmds (or amino 

 acids). Upon strongly acid soils where no nitric acid is formed, these plants, there- 

 fore, have the advantage over autotrophic plants. 



