DISCUSSION OF DEPENDENT PLANTS 377 



denitrification. In discussing the dependent relations of these 

 bacteria we may mention another dependent organism which 

 has recently been described. 1 In the soil a very small animal, 

 closely resembling the amoeba, is said to devour the bacteria 

 of nitrification. This, if true, adds a new feature to what 

 we have known of the interdependency of life in the soil. If 

 these amoeba-like animals become very abundant, they may 

 destroy so many of the nitrifying bacteria that comparatively 

 little nitrification will occur. Hence a smaller amount of 

 nitrites and nitrates will be produced, and hence less nitrogen 

 food from this source will be available for higher plants. But 

 it is asserted that there tends to be a "balance of life," since, 

 if the nitrifying bacteria of any locality are nearly exhausted, 

 the food supply of the animal that lives upon them is thereby 

 so reduced that for a time it does not thrive so well. Mean- 

 time the accumulating compounds offer greater food supply 

 to nitrifying bacteria, and then for a time they may nourish, 

 until when again they are abundant they offer increased food 

 for the organisms that prey upon them. In this way a very long 

 predominance of any kind of organism is usually prevented 

 by the fact that its abundance offers new opportunities for 

 the development of organisms that can use it as food. 



A significant artificial method of regulating the growth of 

 this amoeba-like organism was almost accidentally discovered. 

 Upon the roots of the grapevine there sometimes lives an in- 

 sect parasite known as phylloxera. It was found in France 

 that a treatment of the soil in the vineyards with carbon disul- 

 phide, though an expensive process, would prevent the growth 

 of phylloxera. The fact that after such treatment the soil con- 

 tained more nitrates and nitrites than before led investigators 

 at Harpenden, England, to studies from which they state that 

 the carbon disulphide not only kills phylloxera, but also kills 

 most or all of the amoeba-like animals which live upon nitri- 

 fying bacteria. It does not seriously interfere with the activi- 

 ties of the bacteria that break up the ammonia compounds. 

 1 Hall, A. D., Science, Vol. XXXII, pp. 363-371, 1910. 



