450 Ewart . — The Effects of Tropical Insolation * 
second in sunlight and ten seconds in shade, the leaflets remain 
fully expanded. 
The angle which the leaflet assumes is nicely graduated to 
the intensity of the light to which it is exposed. Thus in only 
moderately strong sunlight the leaves rise up through an 
angle of from 30° to 50°, in weak sunlight from 15 0 to 30° ; 
whilst if the sun is so shaded by clouds that it can barely cast 
a perceptible shadow, the leaflets rise up a few degrees only. 
When the sun’s rays fall somewhat obliquely, but not 
sufficiently so to cause the plane of the entire leaf to readjust 
itself at right angles to the direction of the incident ray, in the 
leaves the axes of which cross the sun’s rays to right or left, 
the further row of leaflets appears to rise up more than the 
nearer one, so that in each pair of leaflets the angles made 
between the incident ray and the distal portion of each leaflet 
is the same, the leaflets apparently responding to the directive 
action of the incident ray. This appearance is, however, really 
due to the twisting of the secondary petiole setting its dorsal 
surface directly facing the sun, for as a matter of fact both 
sets of leaflets move through the same angle in relation to 
a perpendicular line falling between them on the petiolar axis 
to which they are attached. 
In the quite young unopened leaflets naturally no sensitive- 
ness to sunlight can possibly be perceived ; but in young 
opened leaflets of about two-thirds the normal size the 
sensitiveness is even more marked than in adult leaflets, the 
former folding up when exposed to light of less intensity and 
expanding fully horizontally only when very completely 
shaded. 
After prolonged exposure to unbroken sunlight for six to 
eight hours, the leaflets are all completely folded, and re- expand 
in the shade only very slowly. The petioles are generally in 
a more or less drooping position, the loss of water by 
transpiration in spite of the folded position of the leaflets hence 
having been excessive. The leaflets show a rather weak 
power of assimilation, which is in some cases barely per- 
ceptible, but becomes normal again over night. 
