314 BOTANICAL GAZETTE [APRIL 
absorbed. Thus, an increase in the evaporating power of the air or 
in the intensity of impinging solar energy may, with such an instru- 
ment, bring about an actual decrease in the extent of the evapora- 
ting surface and hence a retardation of water loss. While the actual 
rate of water loss remains constant so long as the paper is not 
completely wet, the relative rate of water loss (as the loss from the 
instrument might be compared with that from an open pan of 
water similarly exposed) begins to decrease as soon as the peripheral 
portion of the paper begins to dry out. Of course the drying of 
the disk and the accompanying retardation of relative evapo- 
ration are due to a fall below unity of the ratio of possible water 
supply to water loss; since the rate of possible supply in this 
instrument remains constant, this means an increase in the de- 
nominator of the ratio. A similar result might be occasioned if 
the rate of loss were to remain constant and the rate of possible 
supply were to be decreased. 
This sort of inadequacy in the rate of water supply to maintain 
the original evaporating surface during periods of high evaporation . 
may be postulated as perhaps the main feature in bringing about 
the somewhat sudden fall in relative transpiration observed in the 
early portion of the day. Indeed, a Piche atmometer may be so 
arranged, with an abnormally large, preferably blackened, disk, so 
that if compared to a pan of water or to a porous cup atmometer 
it will exhibit a graph of relative evaporation for the daylight hours - 
quite closely paralleling the corresponding graph of relative tran-— 
spiration for a green plant similarly exposed. 
It is only logically conceivable that there exists, for each leaf at 
any particular time, a maximum possible rate of inward movement 
of water through the petiole, and if the rate of water loss at any time 
surpass this possible rate of supply, the tissues of the leaf should 
become less moist, following the analogy of the Piche atmometer 
described above. This supposed process of drying out of the foliar 
~ cell walls which abut upon the internal atmosphere has been termed 
incipient wilting® by the author last cited. It should take place 
6 Since an incipient process must be regarded as already actually occurring, it is 
quite illogical to apply the term wilting to a condition of affairs which by definition is 
not accompanied by any wilting at all. We have therefore adopted the term incipient 
drying in place of the other term. 
