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 
understood as yet in nature. A number of phenomena, such as the “fairy 
tings” of mushrooms and other fungi, the peripheral growth and central 
decay of lichens, Lecanora, Placodium, Parmelia, and of matforming 
grasses, such as AMfuhlenbergia, 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 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 cuiture formations, the case is quite different. 
The exhaustion of the soil by continuous or intensive cultivation is a matter 
of common experience in aJl 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 
