180 



Humus accumulation is associated with an increase of saprophytic 

 soil organisms, a fact which may be of vital significance in con- 

 ditioning the presence or absence of plants which are dependent 

 on root fungi for their nitrogen supply.* The effect of humus 

 on the toxicity of the soil, while as yet inadequately investigated, 

 may prove to be a factor of large significance. Finally, the 

 accumulation of humus modifies the soil temperature and the 

 air content of the soil. 



Just as the soil factors that influence succession may be summed 

 up under humus, so the air factors may largely be included under 

 the head of shade. Decrease in light, as already demonstrated, is 

 favorable to species tolerant of shade, but fatal to light-requiring 

 species. Moreover, increased shade favors the more rapid 

 accumulation of humus; it also results in increased atmospheric 

 humidity, and hence in decreased evaporation. This latter 

 effect is important, not only in connection with the conservation 

 of soil moisture, but also as it affects the transpiration of the 

 plants themselves. 



Two other biotic agencies are of greater or less importance in 

 their effect on the trend of succession. The first of these is plant 

 invasion. "In the long period of geologic history, plant mi- 

 grations from one region to another must have played a tre- 

 mendous part in the changing aspect of vegetation." But, "so 

 imperceptibly do these migrations take place that we know of no 

 profound change that has been wrought by this means in natural 

 floras within historic time."! To this latter statement it may be 

 that the chestnut blight {Endothia gyrosa var. parasitica (Murr.) 

 Clint.) will furnish an interesting exception. For although much 

 has been written regarding this disease from a pathological and 

 an economic standpoint, it seems to the writer that its possible 

 significance has been overlooked by ecologists. The chestnut 

 is one of the most important trees in the climax forest, not only 

 over a large part of Connecticut, but throughout much of the 



* It should be noted in this connection that recent investigations of W. B. 

 McDougall (Amer. Jour. Bot. i: 51-74. pi. 4-7 +/. i. 1914) would seem to 

 indicate that the benefits accruing to many trees from their association with root 

 fungi may have been greatly overestimated. 



t Cowles, 1911. Op. cit., p. 179. 



