Ewart. — The Effects of Tropical Insolation. 449 
pointing towards the sun remain nearly horizontal or bend 
slightly downwards ; those pointing away from the sun rise 
upwards, and may become nearly vertical if the sun be low 
down but strong and unclouded. The axes of the leaves to 
right and left remain horizontal, but twist so as to face the 
sun, chiefly at the secondary petiolar and partly at the main 
pulvini. The pulvini of leaves in an intermediate position 
partly twist and partly bend in setting the leaf axes in their 
proper position. 
The pulvini of the leaflets respond in a very different 
manner. When exposed to strong and direct illumination, 
they cause the leaflets to rise up and fold more or less com- 
pletely together, thus setting the surfaces of the latter parallel 
to the incident ray; when the light is weak or diffuse they 
cause the leaflets to expand horizontally. The pulvini of the 
leaflets appear to be somewhat more irritable at from 8 to 
12 a.m. than they are later in the day. The leaflets com- 
mence to rise up a second or two after the sun falls upon 
them, and in a half to one minute under favourable conditions 
have risen up their full amount. Owing perhaps to the slightly 
lessened mechanical moment, the secondary petioles may also 
rise up slightly, but this is soon readjusted. If the leaflets are 
shaded after short exposure, they become fully re-expanded 
in from one to three minutes. Occasionally the reactions are 
nearly twice as rapid as the above, whilst after prolonged 
exposure the re-expansion may take place more slowly. If 
the stimulus is removed before the reaction is complete, the 
response continues for a short time in the original direction, 
both a latent period and an after-effect being clearly shown. 
A series of stimuli of short duration may, owing to the exist- 
ence of a latent period, be summated and produce a response. 
Thus, successively exposing to the sun for two seconds and 
then shading for two seconds, finally causes the leaf to rise up 
nearly to the full amount, though it takes a longer time for 
the movement to be completed. If alternately for two seconds 
in sunlight and four seconds in shade, the leaflet rises up, 
though only through about 15 0 to 20°. If alternately for one 
