1910] DACHNOWSKI—ARID HABITATS 335 
and in a direction unfavorable to succeeding agricultural crops. 
No doubt, the “exhaustion” of soils which is frequently met with, and 
which cannot always be attributed to the removal of plant nutrients is, 
in part, an allied phenomenon. It cannot remain a matter of indiffer- 
ence to physiological ecologists whether a strong, intimate, and con- 
trolling relation exists between soil bacteria and surface flora, and how 
the bacterial organisms affect the character, and the association and 
Succession of plants. At best very little is known of this phase of the 
process, and of the reactions and effects of the bacterial products upon 
plant life. It would be idle, also, to expect that the bacteriological 
data in themselves are sufficient for a clear interpretation of toxicity 
and drought relations, or of sterility of soils. If attempted, the inter- 
pretation would be, indeed, one-sided; there is a coordination of 
factors, each and all of which exert a relatively marked influence. 
Temperature, water, and air conditions in the soil, as well as the 
physical and chemical character of it, and the surface flora play an 
important réle in determining the character of the bacterial flora, and 
therefore also the character of the chemical products formed. The 
bacteriological-chemical, as well as the physiological method, deserves 
On that account a closer consideration. The determination of the 
transformation products in various media inoculated with bog bacteria 
Should possess an exactness and a reliability that should make it 
Suitable for the solution not only of agricultural but of ecological 
Problems as well. It is hardly necessary to go into further detail. 
It is only too clear that the need for new investigations is pressing, 
-and that especially new points of view and new lines of research 
are imperatively required. 
rom the point of view of limiting environmental factors, structural 
differences do not seem to play much part in enabling plants to exist 
in bog conditions, for the bog xerophytes, as CLEMENTS contends, are 
dry-land plants, which have retained the ‘distinguishing marks of the 
Original habitat, They are characterized by their ability to adjust 
themselves to changed conditions without undergoing a corresponding 
change of structure. Indeed; drought resistance too often results 
from exceptional vigor and power of endurance rather than from 
Persistence of ancestral or new variant characters. Injury and death 
Yesult only when the condition surpasses the maximal limit. The 
