t911] LIVINGSTON—LIGHT INTENSITY AND TRANSPIRATION 419 
is exhibited in certain of the transpiration graphs of Publ. No. 50 
of the Carnegie Institution. These occurrences may well be due 
to the phenomenon just suggested, which may be termed incipient 
wilting. If the process were carried far enough, actual wilting 
would of course ensue. If incipient wilting be the true cause of 
this sort of fall in transpiration, without any appreciable stomatal 
closure, it should make itself manifest by a gradual fall in the 
gross water content of the leaves themselves, which should be- 
come more marked as evident wilting was approached. It is 
seen, then, that either stomatal closure or this hypothetical in- 
cipient wilting must act to decrease the equivalent evaporating 
surface of plants. By equivalent evaporating surface is here 
meant a free water surface which would evaporate the same 
amount of water as the plant, at the same place and for the same 
period. 
Since the secondary effects of variations in light intensity through 
the photosynthetic process may be safely regarded as negligible 
in the present state of our inquiry, they will not be considered here. 
We may therefore assume that (for short periods having light 
intensities continuously adequate to prevent the closure of stomata, 
and with transpiration rates and a moisture supply which do not 
produce incipient wilting) the plant is virtually to be looked upon 
- 4S an integrating atmometer, automatically summing the various 
increments of water loss from moment to moment as these fluctuate 
in magnitude. It might therefore be expected that a physical 
atmometer exposed at the side of a plant should exhibit the same 
march of the evaporation rate as that evidenced in the transpiration 
of the plant, providing of course that suitable corrections of the 
observed rate be applied, to adequately account for any and all 
internal changes in the organism which were influential in reducing 
the effective or equivalent evaporating surface of the latter. It is 
On this general supposition that the methods used in this study are 
based. 
To keep logically and spatially within bounds, I shall here con- 
sider the effects of changes in the intensity of illumination between 
Strong diffuse light and direct sunshine, thus once for all avoiding 
the question of marked stomatal movement. The stomata in my 
