THE BACTERIA 277 



greatest retarding effect on the growth of these creatures. 

 The action of light in checking the increase of these 

 agents of decomposition and disease is evidently a fact of 

 great practical importance. 



It was stated in Part I. (p. 206) that plants of the 

 Pea and Bean kind, unlike ordinary green plants, are 

 able, by the help of certain fungus-like companions, 

 to obtain their nitrogenous food from the free nitrogen 

 of the atmosphere. The plants in question, including 

 most if not all of our native Leguminosse, invariably 

 have swellings or tubercles on their roots. These 

 tubercles are inhabited by a parasitic or symbiotic 

 organism, the entrance of which into the root is the 

 cause of the first formation of the tubercle. It has 

 been proved conclusively that it is only when this 

 organism is present in the soil that the tubercles 

 develop on the roots, and only when the tubercles are 

 formed that free nitrogen can be assimilated. If the 

 plants are grown, in sterilised soil, i.e. soil which has 

 been heated sufficiently to kill all living things contained 

 in it, then no tubercles develop, and no nitrogen is 

 absorbed from the air. When the tubercles are present, 

 however, great quantities of nitrogen are assimilated, 

 and the plant can thrive even if nitrogenous compounds 

 be quite absent from the soil. A very important result 

 of this fact is that leguminous crops actually enrich 

 the soil in nitrogen. In Germany one sees whole 

 fields of Yellow Lupine grown for no other purpose 

 than to be ploughed in and so enrich the soil for other 

 crops. 



The subject is mentioned here because the organism 

 to which this assimilation of gaseous nitrogen is due, 

 has the appearance of a Bacillus, and has been de- 



