178 
ALFRED DACHNOWSKI AND R. GORMLEY 
tion. The retention of water is vSmaller in plants having very low- 
food reserves, and becomes steadily less as the glycocoU content of the 
solution becomes decreased or the time element in the renewal of the 
culture solution is extended. The progressive decline in weight is 
obviously a pathological manifestation. Experiments are now in 
progress to determine also to what extent the age of plants, their 
resting period, and the upper and lower limit of transpiration, which 
plants of any species rarely exceed, may modify the percentage of 
water retained. Many problems regarding the absorption of CO2 
under these conditions are still in need of future investigation, espe- 
cially since the importance of the atmospheric CO2 gradient gains an 
unsuspected prominence in the supposition that xeromorphy of ancient 
and modern vegetation may be due in part to a modified gas inter- 
change.^ 
To what extent the numerical relation between the quantity of 
water retained and the amount of growth provides a value which may 
serve as a criterion for life zones, yields of crops, and related problems 
of physiology and ecology remains to be determined. The present 
paper is intended to throw some light on the more fundamental 
problem of the "water requirement" (3) of plants during growth and 
metabolism. By way of comparison the essential data of investiga- 
tions on the acid tolerance of these plants have been given in tabular 
form, to enable the reader to draw further conclusions on the acidity 
problem of peat soils. 
No special discussion is required to consider the results, here pre- 
sented, in detail. An examination and comparison of the data will 
lead to the following conclusions: 
1. The value of the transpirational water loss in the experiments 
cited is a function of the vapor pressure of water affected by the 
quantity of the salts in solution, and the factors modifying the atmos- 
pheric conditions. 
2. The transpiration value of plants, when correlated with physical 
conditions of soil solution and atmosphere influencing it, and when 
expressed as a ratio in terms of these or any other factor affecting it 
directly, should be called the ecological water requirement. As such 
it is sufficiently distinctive to characterize diverse plants and diverse 
habitats, and to indicate the limiting conditions and the range of 
deviation of the water relation of vegetation. 
4 Geol. Sur. Ohio Bull. 16: 277. 
