418 BOTANICAL GAZETTE [DECEMBER 
effects of these conditions and by certain responses to other con- 
ditions. One secondary effect of variations in light intensity is seen 
in the opening of stomata when many plants are transferred from 
darkness to diffuse or stronger light. These openings close, or tend 
to close, in many plants when light gives place to darkness or to very 
dim light. But there seems to be no evidence for thinking that 
stomatal movement is at all marked as long as the intensity of 
illumination is above a certain minimum, about what is known 
as weak diffuse light. Of course they close with wilting under any 
light conditions (see Ltoyp, Publ. 82 of the Carnegie Institution). 
Another secondary effect of high evaporation conditions, whether 
caused by direct sunlight or by dryness of the air, etc., is the 
removal of water from the leaves at a rate more rapid than its rate 
of entrance, so that the cells are plasmolyzed and general wilting 
occurs. It is probable that this effect is felt long before actual 
wilting is to be observed; whenever transpiration surpasses the 
rate of water supply to the transpiring tissues it must be supposed 
that a gradual lowering of the vapor tension of the water films 
held in the moist cellulose walls will ensue, just as a semi-dry piece 
of filter paper will exhibit a much lower vapor tension than a similar 
piece saturated. Long before plasmolysis occurs we should expect 
to find that the capillary menisci of the cell membranes abutting 
upon the internal atmosphere of the leaf would become more and 
more concave, and would perhaps break and retreat into the pores 
of the membrane. In the one case, the vapor tension of the water 
film, in the other their actual superficial extent should be reduced. 
It may thus come about that an increase in the evaporating power 
of the air or of solar insolation might produce, by its very accelerat- 
ing influence, a retardation in the transpiration rate. Such a 
phenomenon is common in soils, where an increase in the rate of 
water loss above the rate of diffusion of water to the soil surface 
causes the water films to retreat into the soil and thus decreases 
the rate of water loss. It is thus that the “dust mulch” is produced, 
_ by which adaptation the soil seeks to reduce water loss in a dry 
time! A measurable falling off in the relative transpiration rate, 
occurring in the forenoon, when the evaporating power of the air 
and the light intensity are both still increasing in their daily march, 
