130 SUCCESSION 



have led many to think them the efficient reactions of such 

 successions. It seems almost certain, however, that this is 

 merely a concomitant, and that, even in anomalous succes- 

 sions where facies replace each other without obvious 

 reasons, the reactions are concerned more with water con- 

 tent, light and humidity than with the food content of the 

 soil. 



(6) Success ion by exhausting the soil. 

 This is a reaction not at all understood as yet in nature. A 

 number of phenomena, such as the 'fairy rings" of mush- 

 rooms and other fungi, the peripheral growth and central 

 decay of lichens, Lecanora, Placodium, Parmeha, and of mat- 

 forming grasses, such as MuMenbergia, and the circular 

 advance of the rootstalk plants, indicate that certain plants 

 at least withdraw much of the available supply of some 

 essential soil element, and are forced to move away from 

 the exhausted area. It is probable that the constant shift- 

 ing of the individuals of a formation year after year, a 

 phenomenon to be discussed under alternation, has some 

 connection with this. It will be impossible to establish such 

 a relation, however, until the facts are exactly determined 

 by the method of quadrat statistics. So far as native for- 

 mations are concerned, there cannot be the slightest ques- 

 tion that prairies and forests have existed over the same 

 area for centuries without impoverishing the soil in the 

 least degree, a conclusion which is even more certain for 

 the open vegetation of deserts and plains. With culture 

 formations, the case is quite different. The exhaustion of 

 the soil by continuous or intensive cultivation is a matter of 

 common experience in all lands settled for a long period. 

 Calcium, phosphorus, and nitrogen compounds especially 

 are used up by crops, and must be supplied artificially. The 

 reason for this difference in reaction between native and 

 culture formations seems evident. In harvesting, not 

 merely the grain, but the stems and leaves, and in gardening 

 often the root alao, are removed, so that the plant makes 

 little or no return to the soil. In nature, annual plants re- 

 turn to the ground every year all the solid matter of routs, 



