May, 1922] 
TRELEASE 
— INCIPIENT DRYING AND WILTING 
263 
leavCvS showed a gain in their leaf water content between it a.m. and i p.m. 
And from i p.m. to 3 the leaves gained in water content in one half of his 
observations on maize and milo and in three fourths of those on kafir, w^hile 
from 3 p.m. to 5 the leaves of kafir showed a gain in water in all observations. 
in the case of Cestrum nocturnum, Brown and Trelease (i) found that 
young shoots wilted and actually decreased in length, instead of elongating, 
on dry days during the time they were exposed to direct sunlight; in the 
night such shoots elongated rapidly, but during the day they showed no 
elongation excepting after they had returned to their original length late 
in the afternoon. Absence of growth and actual shrinking were apparently 
connected with excessive transpiration, which caused the plants to lose 
water more rapidly than they absorbed it. The movements of coconut 
pinnae appear to be similar also to the shrinking of tree trunks observed by 
Kraus (5), of fruits observed by Darwin (4) and by Smith (15), and of 
leaves observed by Thoday (16). Thoday found that leaves may shrink 
in area as much as six percent. During periods of intense sunshine, the 
minimum area usually occu.rred at about noon, and alternating periods of 
cloudiness and sunshine during the middle of the day were accompanied 
by prompt increases and decreases, respectively, in leaf area. The re- 
versible movements of coconut pinnae are apparently due to alterations 
in the moisture content of the thin-walled "hinge" cells that lie in a row 
at either side of the midrib, the ''hinge" cells apparently changing readily 
in shape or size, or both, with even slight variations in their water content. 
But it is not to be expected that fluctuations in the average water content 
of the whole pinna would be accurately and promptly reflected in turgidity 
changes and in resulting movements of the hinge tissues. There may be 
a considerable lag between leaf movements and alterations in the general 
foliar moisture, and the hinge cells may be peculiarly sensitive to alteration 
in the relation between transpiration and water supply. The possible 
action of light or temperature as a stimulus, high rates of evaporation, 
or changes in carbohydrate or acid content of the hinge cells or of the 
foliar tissues, etc., may also affect the relationship between the hinge tissue 
and the rest of the pinna. The occurrence of saturation deficit in plant 
tissues follows periods during which transpiration rates have exceeded rates 
of absorption. The physiological importance of such deficits has been 
emphasized by Livingston and Hawkins (8), who point out the possibility 
that the critical value of the ratio of transpiration to absorption (indicating 
the tendency of the plant to have its moisture content reduced) at which 
growth or other vital activity may be definitely afi^ected, may eventually 
become recognized as a physiological and ecological constant, by which 
some of the over-discussed ''adaptations" of plants may be quantitatively 
stated, at least in an approximate way. 
Livingston and Brown (7) have suggested that in the diurnal minimum 
in the water content of foliage leaves we may have a criterion of some im- 
