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The Ohio Naturalist. 
LVol. XIV, No. 4, 
values that produce the effect of wilting and drought, and deter¬ 
mine the differentiation of the vegetation by the local occurrence 
of soil types. It enables to that extent a correlation between avail¬ 
able water and the invasion, succession or reversion, under 
natural conditions, of one vegetation type to another. The 
formula unquestionably provides values which are sufficiently 
distinctive to characterize diverse plants and diverse habitats, 
and which may serve also as a criterion for the range of deviation, 
the maximum and minimum transpiration value for the limits 
of the existence of plants as individuals or as groups, and for 
the geographical distribution of plants where this is determined 
physically by soil, climate or competition. However, correlations 
of transpiration with growth or green and dry weight of plants 
are by no means as clear as they should be; they must be more 
thoroughly tested. 
Critical researches are required in at least three experimental 
fields of investigation to determine (1) how far the observed 
results in growth, structural character, size and weight of plants 
depend on differences in the relation subsisting between absorption 
from the soil and transpiration into the air, (2) how far they 
are due to the differences in the amount of water present and 
retained within the plant, i. e., to differences in the physiological 
water balance in plants, and (3) how far they are determined 
by the biochemical relations of the root-system with the soil- 
water constituents and with metabolism. Here the growth 
increment is the important criterion, and the ratio whiph is 
used as the index of the physiological water requirement (to 
distinguish it from the other term used on the basis of the environ¬ 
mental water relation) may well be called the coefficient of growth. 
To what extent the values of the coefficient may be a measure of 
the relative nutrient efficiency of any salt, or may be determined 
in terms of temperature or of the summation of atmospheric 
factors, i. e., character of climate, and how far they hold out the 
promise of being a standard, mathematically-expressed index 
under soil, seasonal, and plant variations, and how far the range 
of deviation and the minimum value will enable in detecting 
physiological limits to plant processes, to morphogenesis, to 
geographic distribution, or to zonation in montane regions, 
remains to be determined. The problem is decidedly complex. 
It is not the purpose of the present paper to enter into this phase 
of the discussion, but rather to confine itself more closely to the 
relation of transpiration to green and dry substance produced and 
to growth. 
There can be little, if any, doubt that the absorbing power 
of the root system of a plant is not regulated by the amount 
of water transpired, but rather by the differential permeability 
of the absorbing epidermal root cells and the metabolic require- 
