THE BIOLOGY OF SOIL. 63 



in the soil between the absorbing roots of the 

 higher plants and some of these bacteria and 

 fungi, for it has now been established beyond all 

 doubt that certain fungi enter the living roots and 

 there flourish not as mere destructive parasites, but 

 as messmates not only tolerated by the plant, but 

 even indispensable to its welfare. It is probable 

 that nearly half the plants of our fields, moors, and 

 forests entertain such fungi in their root-tissues. 

 The curious, and long-known nodules on the roots 

 of leguminous plants peas, beans, clover, etc. 

 are filled with bacteria which enable these plants 

 to avail themselves of the free nitrogen of the air, 

 and so enrich the soil with nitrogenous substances. 



The roots of most forest trees, orchids, and 

 plants of the moorlands, meadows and marshes are 

 similarly occupied by fungi, which in some way 

 convey salts probably especially phosphates and 

 potassium compounds to the plant in return for the 

 small tax of organic carbon-compounds it exacts 

 from the latter. In some cases at any rate, as 

 Bernard has lately shown, the very existence of the 

 plant depends on its seedling roots obtaining this 

 advantageous attachment and co-operation (sym- 

 biosis) of the fungus immediately on germination. 



These remarks must suffice to illustrate this 

 part of my subject, and to emphasise the statement 

 that the question whether a given plant can be 

 grown in a given soil, is by no means one of 

 simply the physical and chemical constitution of 

 the latter. The plant will have to run the 

 gauntlet of a long series of vicissitudes brought 



