Eiv art . — The Effects of Tropical Insolation . 459 
In Bauhinia , when the illumination is at all oblique, the 
folding of the leaf-lobes may cause the pulvini to be shaded 
and any further movement to cease. 
Both B. elongata and B. tomentosa have an abundance of 
stomata present on the markedly hairy under epidermis of 
the leaf, the upper surface being slightly hairy and possessing 
a few scattered stomata. Hence in the folded position the 
rate of transpiration can only be very slightly diminished. 
In Dalbergia zeylanica , D. linga , Albizza saponaria , Calli- 
andra haematocephala , and Caesalpinia , stomata are present 
on the under surfaces of the leaflets only, the upper epidermis 
having a slightly or markedly thicker cuticle than the under, 
and being without stomata. In Albizza saponaria , Call i- 
andra haematocephala , and Dalbergia linga , it follows from 
the position the leaflets assume, that no checking but rather 
an increase of transpiration is likely to result when the leaves 
are exposed to sunlight. In Dalbergia zeylanica , the leaflets 
of which fold together in pairs, even though no stomata are 
present on the upper surfaces, the folded position of the 
leaflets will interpose a slight check on transpiration ; whilst 
in Mimosa and Acacia , owing to the presence of stomata on 
the upper surfaces and to the overlapping of the leaflets when 
folded, the check must be very marked. In Acacia (165) and 
in Robinia Psendacacia , stomata are abundant on the under 
surface and on the outer half of the upper, becoming fewer 
towards the base of the upper surface of the leaflet, whilst at 
the base none^are present. Hence in these two plants stomata 
are absent from that part in which, when folded together, 
there is least chance of any transpiration taking place. 
The paraheliotropic position is very evidently assumed 
solely in order to protect the chlorophyllous layers of the 
leaflet from exposure to too intense illumination, though it 
may, as a secondary, and perhaps accidental, effect in a few 
cases help to guard against excessive transpiration. 
