458 Ewart . — The Effects of Tropical Insolation . 
assumed their normal position. A leaf exposed for eight 
hours to continuous sunlight showed, if the leaflets were 
allowed to fold together, a fairly active power of assimilation ; 
but another in which, by shading the pulvinus, the leaflets 
were caused to remain nearly horizontal, showed after the 
same time only a moderately active power of assimilation, 
and in parts faint or none ; the next day the remaining 
portions of the leaflets showed fairly active assimilation. If 
one half of the pulvinus be removed, the cut surface being 
smeared with vaseline, the leaflet of that side sinks and can 
no longer respond to a change in the illumination. The 
other leaflet, however, still responds, though not quite so 
markedly as normally. 
B. tomentosa , with a much larger leaf and pulvinus, responds 
more slowly than does B. elongata. When the pulvinus is 
shaded it takes from one to two hours for the two lobes to 
become nearly horizontal, and, when exposed to sunlight, from 
a quarter to a half hour to close up to the full extent possible. 
The leaflets can only close completely together when quite 
young; when older a slight angle (15 0 to 20°) always remains 
between the two leaflets, probably owing to a slight hardening 
of the pulvinar tissue and to the increased elasticity and 
incompressibility of the tissue forming the longitudinal hinge 
between the two lobes. In both B. elongata and B. tomentosa 
the pulvini are covered with brown hairs, and both the epi- 
dermal and outer cortical layers are dark brown in colour. 
This is less marked when the leaf is young, and hence probably 
arises the fact that the quite young leaves fold together 
when exposed to light of only moderate intensity. In the 
older, more resistant, stouter, and less transparent leaf, the 
sensitive pulvinar tissues are largely shielded from the penetra- 
tion of light, and respond, causing the leaflets to fold together, 
only when the light is of considerable intensity and the 
exposure prolonged. The pulvini of the more sensitive 
leaflets of Acacia , Albizza , Dalbergia , and Caesalpinia , are 
all glaucous and green, and are exposed to the sun’s rays 
in almost every position which the leaflets may assume. 
