448 Ewart . — The Effects of Tropical Insolation. 
midday assume a more or less drooping position when exposed 
to full insolation, this being due in the majority of cases not 
to any active movement of the leaf or leaf-stalk, but to 
a general diminution of turgidity, the somewhat flaccid leaf- 
stalk or leaf-base allowing the leaf in virtue of its own weight 
to assume a more or less pendent position. In a large number 
of cases, especially in the Leguminosae, the movements are 
not passive, but are active, the leaves or leaflets being para- 
heliotropic ; i. e. when exposed to intense illumination place 
themselves parallel to the incident ray. 
This phenomenon has already been described by a number 
of observers, but many details are still unsolved. Thus the 
relative irritability of the different parts, the intensity of light 
forming a minimal and an optimal stimulus for movement, the 
relative efficiency of light of different colours, whether the 
position assumed bears any relation to the direction of 
the incident ray, and whether it is the leaf which perceives 
the stimulus and transmits it to the motile pulvinus, or 
whether the latter only is sensitive, being both the motile and 
the percipient organ, are all points requiring an experimental 
answer. 
Mimosa pudica perhaps affords the most favourable subject 
for the experimental study of these questions, owing to its 
marked irritability and sensitiveness, and to its being one of 
the commonest weeds in both Buitenzorg and Peradeniya. 
A curious and interesting fact at once brought to light is that 
the pulvini of the primary and secondary leaves are affected 
differently to the pulvini of the leaflets. Thus the leaf of 
M. pudica reacts as a whole to the directive influence of sun- 
light, the movements of the pulvini being such as to place the 
leaf as nearly as possible at right-angles to the incident rays, 
if the illumination is sufficiently strong. In diffuse daylight 
the entire leaf-system is horizontally expanded, but in the 
morning and late afternoon, when exposed to obliquely falling 
sunlight, the leaf-system sets itself as far as possible at right- 
angles to it, the main petioles and secondary axes twisting or 
bending so that their upper surfaces face the sun. The leaves 
