262 THE BIOLOGY OF FLOWERING PLANTS 



plants in the vegetation probably increases the nitrogen 

 supply to a greater extent than do the soil bacteria. Russell 

 gives an illustration of the effect of clover on a Rothamsted 

 soil. A plot was divided into two portions on one of which 

 clover was grown, on the other barley. In the clover harvest 

 there was removed i5i'3 lb. of nitrogen per acre com- 

 pared with 37*3 lb. per acre in the barley crop. In the 

 soil there was left o*i566 lb. of nitrogen per cent, in 

 the clover plot, and 0'i4i6 in the barley. In the follov.'ing 

 year barley v/as grown in both plots ; in the barley harvest 

 from the former clover plot there was 69*4 lb. of nitrogen 

 per acre, while in the control there was only 39" i. This 

 shows the great importance which leguminous crops 

 have in actual practice. The most valuable results have 

 been obtained in the farming of poor soils, and especially 

 in the use of lupins as a preliminary crop in the reclamation 

 of barren moor and heathland. On such soils innoculation 

 with the suitable organism has been practised with success. 



Biological Races. — Although the symbiotic organisms of 

 all the legumes are included in the one species. Bacillus 

 radicicola, it is certain that this consists of a number of 

 biological races, each capable of infecting only certain 

 groups of species. Hellriegel and Willfarth had shown that 

 the organism of the pea could not infect the lupin, and 

 vice versa. Later work has confirmed and extended this 

 observation. Thus Klimmer and Kriiger (1914) tested by 

 serobiological methods the bacteria isolated from eighteen 

 different leguminous plants and found that they belonged 

 to nine different races ; distinct races, for example, inhabited 

 Lupinus, Vicia sativa, Vicia Faba, and Melilotus, while the 

 last harboured the same race as Medicago and Trigonella. 

 Various attempts have been made to induce the leguminous 

 bacteria to enter into symbiosis with non-leguminous plants, 

 such as cereals, but so far without success. Could such a 

 symbiosis be produced it would have far-reaching practical 

 results. 



Other Plants with Root Tubercles. — This type of symbiosis 

 is not confined to the Leguminosae ; it is found in a Umited 



