54 FROM DUST TO DUST. 



That certain plants, such as peas, beans, vetches, sanfoin, and 

 clover, instead of impoverishing the soil, possessed the power of 

 enriching and rendering it more productive for other crops, has 

 been a well-recognised fact for many centuries. In the earlier 

 Rothamstead experiments, previous to 1861, it had been ascer- 

 tained that while the higher plants, as a rule, absorb no nitrogen 

 from the air, except the small amount to be obtained from nitric 

 acid and ammonia after thunderstorms — the leguminous plants 

 were able in some way to obtain it. Dr. Wilfarth and Professor 

 Hellriegel discovered that their roots, grown in fertile soil, were 

 covered with nodules, and that these nodules were caused by the 

 presence of certain bacteria {B. radickola), which, however, were 

 not parasites, but lived with their plant-host in true symbiotic 

 relationship. They borrow from the plant the necessary hydro- 

 carbons and supply it with nitrogen, which they assimilate from 

 the air circulating in the soil. It has been proved, moreover, that 

 these bacteria are the cause of the nodules appearing on the roots 

 of the plants; for, when the i)lants are grown without the bacteria, 

 they cease to produce these swellings on their roots, which are so 

 remarkably rich in nitrogen and swarming with bacteria. 



Prof. Nobbe confirmed these results, and found that a similar 

 symbiosis exists between microbes and Robinia, and he further 

 discovered that each plant has its own particular microbe, and did 

 not thrive so luxuriantly when the microbe of another plant was 

 substituted for its own. Thus, besides a ge?ieral nitrification of the 

 soil constantly occurring for the maintenance of all plant life, there 

 appears to exist a special arrangement between certain plants 

 and certain bacteria, for their mutual benefit. Whether or no 

 these microbes can be made to adapt themselves to other hosts, 

 in course of time, is being made a matter of experiment by Dr. 

 Schneider, of Illinois. 



Numerous instances are mentioned by Sir John Lubbock, 

 Frank, and others, of symbiosis or commensahsm of certain fungi 

 and some of our forest trees, such as the Beech and Pine, which 

 thrive only when their roots arc in contact with Mycorhiza and 

 other fungi ; and, although it was formerly supposed that the 

 fungi were attacking the roots, it is now considered that the tree 

 and the fungus mutually benefit each other. 



