244 The Ohio Naturalist. IVol. XIV, No. 4, 



values that ijroduce 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 detennined 

 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 detemiine (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 ph^^siological 

 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 which 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 detennined. The problem is decidedly complex. 

 It is not the purpose of the present pai)cr to enter into this ]jhase 

 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 pemieability 

 of the absorbing epidermal root cells and the metabolic require- 



