QUEKETT MICROSCOPICAL CLUB. 261 



have the power of using this gaseous nitrogen as food. Some 

 of these nitrogen-fixing bacteria are independent organisms 

 living in soil or water — e.g. species of Azotohacter. These bacteria 

 do not derive their nitrogen from organic substances, but can use 

 free nitrogen. A second class of nitrogen-fixing organisms lives 

 in the tissues of the higher plants. The most familiar is a 

 small bacterium, Pseudomonas radicicola, which forms the well- 

 known swellings on the roots of leguminous plants. This bac- 

 terium, which already exists in the soil, attacks the seedling plant 

 by penetrating the thin wall of a root-hair. It acts, in fact, as 

 a parasite, and multiplies rapidly. The bacteria form a fila- 

 mentous zoogloea closely resembling a fungus hypha. The 

 infected tissue grows vigorously, and a nodule is formed containing 

 a central mass of parenchyma filled with bacteria surrounded by 

 vascular bundles connecting with those of the root and enclosed 

 by the cortex and epidermis. The parasite phase resolves itself 

 into a condition of symbiosis, the bacteria obtaining their carbon 

 from the sugar stored by the green plant in its root, and also 

 causing the nitrogen and oxygen of the air to combine. Some 

 of the bacteria are digested by the plant protoplasm, which thus 

 obtains a supply of nitrogen. When the plant dies the nodules 

 decay and the bacteria remain in the soil to carry on their work. 

 This explains why a crop of leguminous plants leaves the soil 

 richer in nitrogen than it found it. Similar nodules containing 

 bacteria or fungus hyphae are found in other families. Green 

 plants can ordinarily only derive their nitrogen from inorganic 

 compounds, such as compounds of ammonia or nitrates, which are 

 present in ordinary soils. Apart from the small amount of oxides 

 of nitrogen formed during thunderstorms, these substances 

 are products of the decay of organic matter in the humus, which 

 is broken down by bacteria into simpler substances, among which 

 is ammonia. Some plants can use ammonia compounds as a 

 source of nitrogenous food, but generally they are first converted 

 into nitrates. A bacterium, Nitrosomonas, generally present in 

 fertile soils, converts NH3 into HNO2, which forms nitrites with 

 the bases in the soil. Another bacterium, Nitrohacter, converts 

 the HNOg into HNO3, which similarly forms nitrates. These 

 oxidations will only go on when the soil is properly aerated. 

 Aeration of the soil also checks the action of denitrifying bacteria, 

 which are always present when organic decomposition is taking 



