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1901 | GENETIC DEVELOPMENT OF FORESTS 303 
The humus in the soil is usually variable in amount, but is 
most plentiful where fallen logs have decayed. Hummocks of 
soil rich in humus mark places where dead trees when uprooted 
brought with them quantities of earth. The decaying logs are 
covered with fungi and mosses. Indeed the soil itself is full of 
a living mass of hyphe. The recent researches of Stahl® 
show that mycorhiza is the exception rather than the rule. The 
role that mycorhiza plays is problematic. Some plants, the 
European beech among them, are entirely dependent upon these 
.root-fungi, for when grown in sterile soil they perish. This 
seems to indicate that the beech cannot thrive in soil without 
humus, for its root-fungus is dependent upon organic decay for 
its existence. In other words, the absence of humus means the 
absence of mycorhiza, and its absence excludes the beech. 
Indeed, future investigation may show that a large majority of 
forest trees and other forest plants have established an obligate 
mutualistic relation with root-fungi. 
THE DUNE sociETIES. In discussing the sand societies the 
conditions favorable to the formation of dunes have been dis- 
regarded. Where the wind sweeps across the fossil beach the 
succession of plant societies is retarded. The extreme exposure 
to the xerophytic influence of the wind often prevents the pres- 
ence of plants. Nevertheless, when not too severe, a definite 
beach society is finally attained. The origin and development 
of dunes and dune societies have been clearly shown by Cowles.? 
It is only necessary to emphasize the fact here that the plant 
succession on dunes is similar to that on the beaches, and that 
in time the climax maple-beech society is reached. Fig. 1 is 
from a photograph, showing a fossil beach society in the fore- 
ground. On the right is a dune clothed with a coniferous forest. 
At the base of the dune is a heath encroaching upon the beach. 
On the landward side of this dune there are indications of the 
beginnings of a maple-beech forest, and farther inland are dunes 
*STAuL, E.: Der Sinn der Mycorhizenbildung. Jahrb. f. wiss, Bot. 34 : 539-668. 
1goo. 
°Sand dunes of Lake Michigan, Bor. Gaz. 27:95 ff. 
