Ewart . — The Effects of Tropical Insolation . 453 
from above, but some illuminated from beneath by sunlight 
reflected from flat mirrors, the leaflets, the pulvini of which 
are illuminated, rise up, the others remaining horizontally 
expanded. If the light is somewhat concentrated by means 
of a plano-concave mirror, the leaflets re-act nearly as actively 
and rise up nearly as much as when exposed to direct sunlight 
falling from above. 
Light therefore exercises no directive influence upon the 
pulvinus of the leaflet ; it is only a change in the intensity of 
the illumination which acts as a stimulus, the movement 
always taking place in the same direction, no matter how the 
light falls upon the pulvinus, and the angle through which 
the leaflets move being dependent upon the intensity of the 
stimulus. 
As is well known, when the intensity of the light to which 
the plant is exposed falls below a given intensity, the leaf 
assumes the nyctitropic position ; the primary and secondary 
petioles drooping whilst the leaflets rise up and fold together. 
When the evenings are bright and cloudless, the leaves begin 
to close in the tropics after 5 p.m., and by 5.30 p.m. are quite 
closed, a rough calculation showing that they thereby lose 
about the equivalent of five to ten minutes 5 bright midday 
illumination for assimilating purposes, which is a trifling and 
perhaps negligible amount. The folding together of the 
leaflets, along with the diminished temperature and closure of 
the stomata, besides hindering or preventing assimilation, also 
interposes a marked check upon transpiration, causing the 
plant, and more especially the pulvinus, to be temporarily in an 
even more highly turgid condition than is normally the case. 
If, when the full nyctitropic position has been assumed, the 
main pulvinus of the leaf is stimulated by touching the 
sensitive hairs on its under surface, it gives a marked response, 
and the turgidity of the upper surface of the pulvinus may be 
so much greater than that of the stimulated and originally 
convex surface that the leaf is bent beneath or across the 
stem, thus reversing the normal position of dorsal and ventral 
surfaces, and is supported against the action of gravity in this 
