262 THE FORMATION 



roots in breaking up the soil particles, and in changing them into soluble 

 substances. Mycorrhizae, bacterial nodules, and especially soil bacteria play 

 a large part in increasing the nutrition-content of the soil, but the extent to 

 which they are effective in succession is completely unknown. The changes 

 in the color, texture, and food value of the soil in passing from the initial to 

 ultimate stages of a normal succession are well known, and 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 anom- 

 alous successions where facies replace each other without obvious reasons, 

 the reactions are concerned more with water-content, light, and humidity 

 than with the food-content of the soil. 



321. Succession by exhausting the soil. This is a reaction not at all 

 imderstood as yet in nature. A number of phenomena, such as the "fairy 

 rings" of mushrooms and other fungi, the peripheral growth and central 

 decay of lichens, Lecanora, Placodium, ParmcUa, and of matforming 

 grasses, such as Muhlenhergia, and the circular advance of the rootstalk 

 plants, indicate that certain plantj 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 shifting of the in- 

 dividuals 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 formations are concerned, 

 there can not be the .slightest question 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 be- 

 tween native and culture formations seems evident. In harvesting, not 

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

 also, are removed, so that the plant makes little or no return to the soil. In 

 nature, annual plants return to the ground every year all the solid matter 

 of roots, stems, leaves, and fruits, with the exception of the relatively small 

 number of seeds that germinate. Perennial herbs return everything but 

 the persistent underground parts. Shrubs and trees replace annually an im- 

 mense amount of material used in leaves and fruits, and sooner or later, by 

 the gradual decay of the individuals or by the destruction of the whole 



