Ewart . — The Effects of Tropical Insolation. 457 
surface of the pulvinus, which is uppermost in the sunlight- 
position, has a moderate amount of red dye in the epidermal 
cells covering it. Stomata are abundant on the under surface 
of the leaf, but none are present on the upper. Hence the 
movement is evidently a means of protection against light 
alone and not against excessive transpiration ; for if the latter 
were the case we should expect to find the ventral stomatic 
surfaces being apposed to one another. The outer surfaces of 
the epidermal cells of the leaflet are curved in outline, the 
curvature on the upper surface being more marked and 
conical, on the under flatter and more rounded. This would 
probably indicate according to Stahl 1 that Cassia montana 
is specially adapted to suit a shady habitat. The sunlight 
position which the leaflets assume, though sufficient as 
a protection against not too prolonged exposure, is, owing 
to the way in which the ventral surfaces are exposed, 
not so adequate a protection as in the other cases already 
described. 
The bilobed leaf of Bauhinia , though slow to react to 
sunlight, forms nevertheless, owing to the large size of the 
pair of leaflets of which it is composed, to the manner in 
which these are hinged together in the median line for a 
great part of their length, and to the pulvini being partially 
bifid, interesting material for experimental study. In B. 
elongata the leaflets, when exposed to full sunlight, fold 
almost completely together, each rising up through an angle 
of 8o° or more. If the pulvinus at the base of the pair of 
leaflets be shaded, the area shaded being about 1 sq. cm., 
whilst the area of the leaf left exposed is from 100 to 200 
times greater, in a quarter of an hour the leaflets are seen to 
be expanding, in half an hour they make an angle of 15 0 
to 20 0 only with the horizon, and in one hour they are nearly 
horizontal. On then re-exposing fully to sunlight, or on 
exposing the pulvinus alone, the leaflets begin to rise in five 
minutes ; in ten minutes they have nearly, and in fifteen fully, 
1 Stahl, Ann. d. Jard. Bot. de Buitenzorg, Vol. xiii, 2 e partie, 1896. ‘ Sammet- 
bliittern,’ &c. 
