1912] LIVINGSTON & BROWN—TRANSPIRATION 329 
pletely to exhibit a diurnal fall in foliar moisture under conditions 
of evaporation which render it manifest in the common type of 
thin-leaved plants (such as Martynia, Sida, Physalis), as well as 
in such pronounced succulents as the Portulaca-like Trianthemum 
of our work. It is suggested that these exceptional small-leaved 
xerophytes may actually show a somewhat higher leaf moisture 
content by day than by night, but this proposition is uncertain. 
While the other logically possible cause of this diurnal decrease 
in relative water content of foliage leaves, namely, a diurnal 
increase in materials other than water within the tissues, remains 
still to be considered in a thoroughly adequate way, our findings 
fail to adduce evidence in favor of this as the true cause of the 
observed phenomena, and do furnish several lines of indirect 
opposing evidence. It may be stated, therefore, that, so far as 
evidence is at hand (including indirect considerations of the litera- 
ture, not here cited), it is probable that the cause of this diurnal 
minimum in foliar moisture rests in the phenomenon of incipient 
drying, brought about whenever the ratio of water loss to water 
supply in the leaves is rendered less than unity. It may thus be 
suggested that, although our tests with Physalis would lead to the 
conclusion that the external factor which controls this diurnal fall 
of leaf moisture is evaporation intensity simply, the true control- 
ling condition is more probably the ratio of water supply to water 
loss. Thus, the structure of the plant (including all of its various 
“adaptations” to dry habitats), the moisture conditions of the 
soil, intensity of evaporation and of solar illumination appear to 
make up the controlling environmental complex. 
It seems highly probable from our studies that the diurnal, 
non-stomatal retardation of the escape of water vapor from green 
leaves in sunlight (as first described in Publ. 50, Carnegie Inst., and 
there attributed to the influence of temperature or evaporation 
intensity) is but the effect of a lower vapor tension within the 
internal atmosphere of the leaves and over their surfaces, this lower 
vapor tension being brought about by the increased surface tension 
and decreased evaporating surface which accompanies a lowered 
water content of the internally and externally exposed cell walls. 
In conclusion, it may be suggested that we have here, in- the 
