90 BACTERIOLOGY 



form by the action of saprogenic organisms of the soil. These 

 attack the organic animal and vegetable matter such as manure, 

 decaying leaves, etc., and break up their protein constituents 

 with the formation of, amongst other substances, Ammonia, 

 which combines with potassium and sodium and other in- 

 organic elements in the soil to form Ammonium Salts of 

 these substances. Certain bacteria then have the power of 

 causing the oxidation of these ammonium salts to nitrites. 

 Another set of bacteria effects the oxidation of the nitrites 

 into nitrates, the latter of which can now be absorbed and 

 utilised by the plants growing in the soil. It is interesting 

 and important to note here that certain other bacteria have 

 the power of de-oxidising or reducing these nitrates, and, if 

 these are present in a soil in great numbers, their action is 

 very harmful, as they de-nitrify it, and thus undo the bene- 

 ficent action of the nitrifying group described above. 



Nitrogen-fixation by Soil-Bacteria and by Root-Nodule 

 Bacteria. Another important means by which nitrogen is 

 supplied to plants is by the instrumentality of another group 

 of bacteria which can seize upon the Free Nitrogen of the Air 

 and by various chemical processes transmit it to plants. Some 

 of these bacteria are free in the soil, but another very interest- 

 ing group has the peculiar property of entering the roots of 

 leguminous plants such as clover, beans, peas, lucerne, vetches 

 and the like, giving rise to tumour-like swellings known as 

 Root-Nodules a sort of symbiosis, or " living together '' in a 

 partnership beneficial to both plant and bacterium. Interest- 

 ing experiments have been carried out by growing such legumin- 

 ous plants in soils respectively rich and poor in these bacteria 

 or completely sterilised by heat and so bacteria-free. The 

 plants grown in the former flourish and outgrow those in the 

 latter, which often remain stunted and poorly developed. 

 Much was at first hoped for by the artificial use of these 

 bacteria as soil-fertilisers, but the actual results so attained in 

 practice have as yet been somewhat disappointing. The ro- 

 tation of crops, with the periodic sowing of nodule-bearing 

 leguminous plants, such as clover, or vetches, is the usual 

 method by which such bacteria and their resulting nitrogen 

 are increased in amount in the exhausted soil. 



Bacterial Diseases of Plants. Plants, no less than animals, 

 are subject to various bacterial, protozoal, and other forms of 

 infective disease. Various Blights and Rots, Wilts, Leaf- 

 Spots, and even Tumour-like Nodules and Excrescences may 

 be caused. Pear-Blight is caused by a motile bacillus, B. 

 amylovorus, which is probably carried from infected trees and 



