456 Ewart . — The Effects of Tropical Insolation. 
former become markedly expanded in a quarter of an hour, 
and in half an hour are nearly horizontal. If fully exposed 
to sunlight, the leaflets rise up or twist to their full extent in 
ten to twenty minutes. The leaflets of several species of 
Caesalpinia , of Dalbergia linga , and of Desmodium velutinum , 
D. C., twist, so that the longitudinal axis is at right angles 
to the lamina or parallel to the sun’s rays, the proximal edge 
of the leaflet being uppermost. At the same time the 
leaflets may in some cases rise somewhat, but the main and 
secondary petioles generally droop more or less after prolonged 
exposure. In this case also the movement of the leaflets is 
always the same no matter what the angle at which the 
incident rays fall, the intensity of the light alone acting as 
a stimulus to movement. Likewise, if the pulvini alone are 
shaded, the flat surfaces of the leaflets become horizontal, 
the dorsal (morphologically ventral) surface being directed 
upwards ; but if at the same time the leaf be powerfully 
illuminated from the side or from beneath, the leaflets, though 
shaded from the sunlight falling from above, retain their 
paraheliotropic position. 
In Albizza sp. ? when exposed to sunlight the leaflets fold 
forwards and also twist so that the proximal edge is upper- 
most and the lamina vertical. If the pulvini are shaded, the 
leaflets begin to expand and twist in five minutes and attain 
nearly to their normal shade position in fifteen minutes. 
Albizza saponaria , Bl., and Calliandra haematocephala , Hsskl., 
are similar in all respects, with the exception that the move- 
ment is slower and takes nearly twice as long before it is 
completed. 
The leaflets of Cassia montana , Heyne, when exposed to 
sunlight fold together forwards, more or less, whilst the 
pulvinus twists so that the leaflets droop downwards with the 
under ventral surface directed outwards and in some cases 
slightly upwards. If the pulvini are shaded, the leaflets begin 
in ten minutes to expand, and rise up and twist so that the 
dorsal surfaces face upwards, a nearly normal shade-position 
being assumed after twenty to thirty minutes. The proximal 
