454 Ewart — The Effects of Tropical Insolation . 
position for some time. The same stimulation applied to the 
expanded leaf at midday causes it merely to fall and droop 
downwards, the fall being largely a passive one, due to the 
action of gravity being no longer counteracted by the greater 
turgidity of the convex under-surface of the pulvinus. It must 
not however be concluded that the sleep-position assumed in 
the evening is simply due to the general increase of turgidity 
consequent upon the diminished transpiration. The folding 
up of the leaflets, which is one of the main factors causing the 
diminished transpiration, is directly due to the stimulating 
action of the fading light ; for any general increase in turgidity 
tending to make that of the upper surface more nearly equal 
to that of the under surface of the pulvinus would tend, not to 
close the leaflet, but to keep it expanded, were it not over- 
powered by a more powerful direct stimulus. Besides, were 
the latter not the case, we should expect to find the leaflets 
expanding during the night as the normal relation between 
transpiration and absorption became restored ; whereas, as is 
well known, they remain folded together until caused to 
expand by the early morning light. The fall of the main 
and secondary petioles may, however, partly be brought out 
in the manner above described, for they rise somewhat during 
the night ; but even here the movement is mainly due to the 
direct stimulating action of the absence of light, for it is only 
on re-exposure to light of adequate intensity that the normal 
fully-expanded condition is assumed. 
Equally exposed plants of Mimosa growing in the one case 
in dry sandy and in the other in damp humid soil, close at 
the same time in the evening, and not earlier in the latter 
case than in the former. If a portion of a plot of plants be 
kept dry and the rest well watered, with equal exposure, and 
the plants being to commence with fully expanded, the leaves 
and the plants in the latter may close five to ten minutes earlier 
than in the former ; but this simply shows that the more 
turgid plants respond somewhat more rapidly to the stimulus 
due to the fading intensity of light than do the less turgid 
ones. 
