68 PLANT PHYSIOLOGY 



bodies of animals and plants. These serve as the foods or 

 as the food-materials of plants of various degrees of depen- 

 dence, e. g. certain species of orchids, the so-called sapro- 

 phytes, etc. Plants of this sort we shall consider later 

 (pp. 78-92). 



The main source of nitrogen for independent plants, that 

 is, for plants which contain chlorophyll and which by means 

 of it elaborate their own non-nitrogenous foods, is the salts 

 of nitric acid, which are found in the soil and in water. 

 The origin of these compounds in the soil, once supposed to 

 be purely chemical, has only recently been shown, by the 

 brilliant researches of Winogradsky, * to be the result of 

 the activity of micro-organisms universally present in soil 

 These organisms, bacteria of only a few species and of ex- 

 ceedingly small size, have been isolated and cultivated by 

 Winogradsky. He found them to be of two sorts, strikingly 

 different in their physiological activity. The one sort to 

 which Winogradsky gave the generic names Nitrosococcus 

 and Nitrosompnas oxidize ammonia compounds (e. g. 

 ammonic sulphate (NH 4 ) 2 S0 4 ) to nitrites, salts of nitrous 

 acid. The other sort belonging to a single genus, called by 

 Winogradsky, Nitrobacter oxidize these to nitrates, salts of 

 nitric acid. In the soil there is no accumulation of nitrous 

 acid or its salts, for they are as rapidly oxidized by Nitro- 

 bacter as they are formed by Nitrosococcus and Nitrosonio- 

 nas. Furthermore, there accumulates no free nitric acid, for 

 this is neutralized by the carbonates commonly present. 

 These reactions may be represented in simplest form thus : 

 (NH 4 ) S0 4 + 3 2 = 2 HN0 + H 2 SO 4 + 2 H 2 O 



2 HNO 2 + 2 = 2 HN0 3 

 2 HNO 3 + K 2 C0 3 = 2 KN0 3 + C0 2 + H 2 O 



As already suggested (see p. 20), these nitrogen bacte- 

 ria are strikingly different from all other known organ- 

 isms. Though they obtain the carbon needed for food- 

 manufacture directly from the air as carbon-dioxide, they 

 elaborate this without chlorophyll or bacteriopurpurin ( see 



* Winogradsky, S. Recherches sur les organismes de la nitrification. 

 Annales de 1'Institut Pasteur, IV., V., 1889-91. Zur Mikrobiologie des 

 Nitrifikationsprozessee. Centralblatt f. Bakteriologie, 2te Abth., II., 1897. 



