1911] COWLES—V EGETATIVE CYCLES 177 
It also offers a fascinating field for study. The depletion of mineral 
foodstuffs in the soil has been urged as a successional factor, but 
it is doubtful if this is of any consequence. The great abundance 
of the mineral constituents of plants in nearly all soils is in strong 
contrast to the minute amounts which the plants contain. Further- 
more, the plants in their decay return to the soil the mineral ele- 
_ ments which they took from it. 
e. Temperature and aeration—Finally humus accumulation 
alters the soil temperature and the air content of the soil. For 
the most part changes in air content and in temperature probably 
are insufficient to be of great influence in vegetative change. In 
bogs, however, there is evidence that each of these factors is of 
importance. TRANSEAU has shown (24) that in the growing season 
the temperature of the water and of the soil in bogs is below that 
of other soils, and of the superincumbent air. Such a condition 
certainly is detrimental to root activity. Similarly TRANSEAU 
(24) has shown that the lack of aeration in bog soils is detrimental 
to root activity. Thus for these reasons (and probably also because 
of soil toxicity, as noted above) certain stages in bogs are char- 
acterized by the development of a xerophytic vegetation, since the 
unfavorable conditions for root absorption make existence in bogs 
difficult for any plants with aerial organs except such as have 
structures which reduce transpiration. That such bog plants are 
actual and not merely apparent xerophytes was demonstrated in 
brilliant fashion by TransEav (24), who produced plants with 
Xerophytic structures from ordinary plants by growing them in 
§ Conditions. 
C. SHADE 
Next in importance to humus among the dynamic biotic agencies 
's shade. The foresters have known for generations that in the 
teloresting of a region the first trees to appear are those which 
Fequire a large amount of sunlight for their development; con- 
‘picuous among such light-requiring pioneers are the poplars and 
itches. Rarely is a dense growth of these trees followed by trees 
of similar kind, since the increasing shade makes the development 
ot seedlings of these species more and more difficult. Other trees, 
however, perhaps pines and oaks, are able to thrive in-a degree of 
