440 Ewart. — The Effects of Tropical Insolation. 
confirmatory observations were made. The methods em- 
ployed were the same as those already described in previous 
papers, the evolution or non-evolution of oxygen, as detected 
by the Bacterium- method, under optimal illumination, being 
taken as a test for the presence or absence of the power 
of assimilation. 
It might at first sight appear probable that under the 
intense tropical sunlight all fully exposed green leaves, should 
suffer Considerably ; but, as a matter of fact, so long as the 
plant is abundantly supplied with water and is kept cool 
by transpiration, it suffers no more under such exposure than 
do plants in temperate regions. It may indeed be concluded, 
with a fair amount of certainty, that practically in all tropical 
plants which normally grow in fully exposed situations, pro- 
vided the leaves are healthy and abundantly supplied with 
water, no assimilatory inhibition directly due to insolation 
ever takes place under normal conditions of exposure. Thus 
the healthy adult leaves of the following plants, after a full 
day’s exposure to perpendicular solar illumination, were 
green, fresh, and showed an active power of assimilation: 
Caryota maxima , Bl., Musa Ensete , Kentia McArthuri , 
Alocasia in die a, Schott., Artabotrys Blnmei , Uncaria sclero- 
phylla , and Areca .Catechu, L. ; whilst in Bauhinia , Caesal- 
pinia , Acacia , and Dalbergia ling a, if the leaves, owing to 
their paraheliotropic irritability, are allowed to droop or fold 
together, no appreciable effect is produced ; but if they are 
kept fully exposed to perpendicular illumination they become 
much paler green in colour, are more or less wilted, and 
the power of assimilation is weakened, diminished, or in- 
hibited, temporarily or permanently. 
There are a variety of ways in which the leaves of exposed 
tropical plants may be rendered capable of withstanding 
excessive insolation. Perhaps the commonest, though the 
least obvious, is by means of some invisible inherent organic 
change or difference in the chlorophyll-bodies of plants 
growing in exposed situations, as compared with those fre- 
quenting shady habitats. Along with this go changes in the 
