349 | B. H. Invingston 151 
atmometer, which has also been devised and perfected here, 
and this apparatus is likewise coming into general use. 
The two conditions just mentioned are both effective from 
without the plant; they are external conditions. There is also 
an internal condition (effective from within the plant) that 
exerts great influence upon the rate of evaporational water- 
loss from plant surfaces, and this has been called, in our dis- 
cussions, the transpiring power of the plant. Studies largely 
carried out in this laboratory have resulted in the perfection 
of methods by which the intensity of this internal condition 
may be evaluated, and integrated over convenient time periods. 
Reference is here made to the method of relative transpiration 
and to that of cobalt-chloride paper. Both are now frequently 
employed in studies of plant growth. 
As has been mentioned, the rate of water-supply to the 
plant also requires attention in studies of the water relations. 
This is primarily the water-supplying power of the soil, 
another external condition. The need for quantitative study 
of this has led to investigations in the realm of soil physics, 
and our efforts have already resulted in some useful methods 
of approach, but more work will be necessary before we can 
deal with this subterranean condition as satisfactorily as is 
now possible with the conditions that are effective above the 
soil. Out of our work has come the auto-irrigator, an instru- 
ment which maintains the moisture conditions of the soil 
nearly constant throughout long periods of time. Its readings 
indicate the rates at which an experimental plant removes 
water from a soil mass thus automatically supplied with 
water. The auto-irrigator is now employed by many experi- 
menters, in cases where it is desired to maintain a constant 
soil-moisture content. Soil osmometers have also been em- 
ployed in the study of the water-supplying power of the soil as 
related to absorption by plant roots. 
By the employment of these various methods, all perfected 
here within the last few years, we have been able to begin to 
understand some of the more fundamental features of the . 
