1910] DACHNOWSKI—ARID HABITATS 320 
knowledge of the limits of functional variation within species, there- 
fore, is very essential. Deviations in the very functioning of the pro- 
cess, and in transmission as well, cannot but be fruitful in a greater 
efficiency of responses in plants, and without doubt these are the 
differences that go far toward constituting the essential difference 
between relative power of endurance and resistance against drought, 
disease, or abnormal conditions in any plant. The circumstances 
under which bog plants can function unaltered, either in character or 
degree, and the limits beyond which such functions must cease or 
become altered, are questions of special significance in physiological 
ecology. ‘They throw no little light upon the nature of tolerance and 
drought resistance,'a topic upon which information is sorely needed 
if we are to regard functions no less important in the light of develop- 
ment and heredity than form or structural organization. 
Of still greater consequence is the study of such limiting factors 
of soil and climate as operate to preserve adapted forms, or eliminate 
species in which the responses fail to secure better adaptation to the 
factors provided by a local region. The presence and fitness of bog 
plants and of invading hydrophytes and mesophytes resistant to 
Physiological drought seem to be due mainly to more efficient 
Tesponses to edaphic conditions. In most cases, at least, drought 
resistance involves a specific reaction on the part of the plant to more 
than one factor of the physical and biotic environment, the ratio 
between the factors in the process being more significant than the 
structural modifications or the regulatory compensation which may 
be possible within the plants. The reaction may be in part one of 
structural differences, and in part one of endurance. The evidence 
cited in the work published, and experiments now in progress indicate 
that drought resistance in physiologically arid habitats is due toa 
Specific protective functional root activity. It is fundamentally a 
chemical reaction, but one too complex to solve by ordinary methods 
of chemistry. The delicacy of the reaction may be better understood 
if we recall the fact that it is adjusted to meet a specific limiting con- 
dition. Whether the toxic action of bog water and bog soil is deter- 
mined by the action of one constituent, or by the combined action 
of several, needs more detailed study. As has been noted elsewhere, 
the toxicity of any bog soil is but a function of the number of the con- 
