VI The Web of Life 113 



the salts of the soil ; many animals feed upon plants, even 

 the most purely carnivorous are of course indirectly depend- 

 ent upon them ; dead animals are consumed by bacteria, 

 which in so doing set free carbonic acid gas and ammonia, 

 which steal off into the air, and nitrates and the like which 

 soak down into the soil, all to be utilised by the plant- 

 world anew. Here then the bacteria furnish the indispens- 

 able means by which the circulation of the materials of 

 organic life is perpetually renewed. 



In this connection we may appropriately discuss an 

 interesting story in regard to the relations between bacteria 

 and some other plants. Pull up a bean-plant, wash gently 

 and examine the roots. Here and there may be noted 

 small rounded swellings or tubercles, sometimes as large 

 as peas. The same may be seen on the roots of vetches 

 and other leguminous plants, and also on some cereals. 

 That they are not normal parts of the plants is clear, for 

 there are roots without a trace of them ; and if the seeds be 

 grown in soil that has been calcined at a high temperature 

 or in water that has been boiled, the tubercles are not 

 formed. This becomes yet more evident when the tubercles 

 are examined microscopically, for then they are seen to 

 exhibit, not the tissues of a rootlet, but crowds of minute 

 rod-like or somewhat spherical bodies. 



What then is the meaning of the tubercles ? The re- 

 searches of numerous botanists, with a controversy which has 

 been in progress since 1858, when these tubercles were first 

 noted, have at length convinced most of us that the little 

 rod-like bodies contained in the tubercles are bacteria, and 

 that the tubercles are due to the infection of the plants by 

 these micro-organisms. As to their physiological effect on 

 the plant, the investigators are not yet thoroughly agreed, 

 but the conclusions of Hellriegel and Willfarth, corroborated 

 by some others, are of great interest. By growing legum- 



I 



