Ewart . — The Effects of Tropical Insolation. 445 
that the leaves of such plants as Hoya, though probably their 
resistant powers in this respect are above' the normal, have, 
when fully insolated, their assimilatory powers largely dimin- 
ished by the high temperature which they attain, and may, 
when subjected to prolonged exposure, have their power of 
assimilation temporarily or permanently affected, partially or 
mainly from this cause alone. This is the case in the following 
experiments. 
Vanilla aromatica. After ten hours’ exposure, leaves paler green ; 
yield weaker extract of chlorophyll than normal ; assimilation weak 
or absent in palisade-parenchyma, weak to nearly normal in spongy 
mesophyll. Next day assimilation active, except in few parts where 
faint or absent, the latter finally dying. Preparation difficult owing 
to presence of mucilage and raphides. Full insolation-temperature 
is about 40°C., occasionally rising to 45 0 C. 
Hoya fraterna. Similar results to those with Vanilla , but the 
inhibition is more complete ; the leaf is pale green to pale yellowish 
or brownish green, and in latter cases none or only a partial recovery 
takes place. Insolation-temperature 45 0 C. occasionally rising to 50° C. 
Leaves in exposed situations soon become yellowish, and assimilation 
is weak or absent. If kept in weak diffuse daylight, provided the 
change of colouration was not too pronounced, they become green 
again in one to two weeks, but show active assimilation long before 
this. 
A cut stem of Vanilla , 4 ft. long, left entirely without water, 
remained living for a long period of time, the power of 
assimilation in the leaves being after one week fairly active, 
after two faint, and after three absent. But in the last case, on 
placing such a leaf with the cut surface in water, after two 
days a weak power of assimilation was shown. The stem and 
leaves began to wilt basally upwards after six weeks ; after the 
seventh week a small leaf and root produced at the apex after 
separation from the parent plant ceased to grow ; the leaves 
were thin and shrunken, the cells in the basal leaves dead 
and unplasmolysable, though the colour was but little altered. 
After ten weeks all the leaves are dead. Vanilla is therefore 
H h 2 
