446 Ewart . — The Effects of Tropical Insolation . 
very resistant to drought, and any effects produced by insola- 
tion are not due to any excessive loss of water, but to the 
heating and photo-chemical effect of the sun’s rays. 
In the young green leaves of Hoya fraterna , when \ to | full 
size, minute starch-grains are present in the chlorophyll- 
corpuscles, and later, several larger ones are present in each ; 
but it is only after the leaf is adult, and has been actively 
assimilating for some time, that the characteristic large oil- 
drops (soluble in ether and turning brown with osmic acid) 
appear in the assimilating cells. If such leaves are kept in 
darkness, in five days the oil-drops disappear in almost all 
the cells, but the starch is not perceptibly, or only very 
slightly, diminished in amount, whilst in ten to fifteen days 
almost every trace of starch has also disappeared. The proto- 
plasm hence first absorbs and uses up the stored oil, and later 
the chlorophyll-grains yield up to it their stored carbohydrate. 
The slower disappearance of the starch is not due to the 
chlorophyll-grains being sick, for on examination after fifteen 
days in darkness a fairly active power of assimilation is shown. 
In Dicranum scoparimn , Musa , Vaucheria , &c , the assimilatory 
products are at once converted into oil, starch-grains appearing 
in the chloroplastids only when a superabundance of assimi- 
lated material is formed. In Hoya , on the other hand, oil is 
formed only when an excess of assimilated material is present; 
it is the last to appear and the first to disappear. This 
peculiarity enables an interesting corroboration to be made 
of the results already obtained with Hoya. Thus in leaves, 
in which, without any fatal injury being inflicted, the power 
of assimilation is inhibited or reduced to a minimum by 
continued over-exposure, the oil originally present may 
entirely or almost entirely disappear. 
In a previous paper 1 it has been shown that any agency 
which, when applied in intense or concentrated form, directly 
inhibits or prevents assimilation from taking place, may, when 
the application thereof is much weaker but prolonged, gradually 
1 Loc. cit., Journal of Linnean Society, 1896, pp. 364, 461. 
