444 Ewart . — The Effects of Tropical Insolation. 
exposure and not return. Young leaves are red till half grown, and 
show normally weak to moderately active assimilation ; but after six 
to eight hours’ exposure weak or none, assimilation in some cases not 
returning, but in others in one day being normal again. 
The experiments with Dendrohium show that localized 
exposure produces a purely localized effect. With the stoppage 
of assimilation a change of colouration is generally asso- 
ciated, but is not an essential accompaniment thereto, for 
the power of assimilation commonly returns before any per- 
ceptible regeneration of the original intensity or shade of 
colouration takes place. The results obtained with Pisonia 
alba indicate that the effect produced is a photo-chemical one, 
and is not primarily due to, though it may undoubtedly be 
aided by, the heating effect of the sun’s rays, the latter being 
the case whenever the leaf becomes at all wilted. It very 
commonly happens that when a leaf is exposed, the shaded 
side may retain, especially if the leaf is a thick one, a distinct 
power of assimilation, even though the palisade-layers com- 
posing the upper exposed surface are almost fatally affected. 
If a bifacial leaf is exposed, under surface upwards, it is much 
more likely to be fatally affected than when in the normal 
position. 
Thick fleshy leaves do not wilt when exposed for prolonged 
periods ; but, owing to the relatively slight amount of tran- 
spiration of which they are capable, they are very liable to be 
overheated by the absorption of the solar heat-rays. Thus in 
Hoya fraterna the insolation-temperature of the leaf is about 
45° C., occasionally rising to over 50° C. The temperature was 
measured by suddenly bending the leaf around and pressing 
it against a delicate thermometer which was originally a little 
below the expected temperature. Since the general optimum 
temperature for assimilation probably lies between 25 0 C. and 
35 0 C., steadily diminishing from 40° C. onwards, until at 50° C. 
or thereabouts the maximal temperature is reached 1 , it follows 
1 See Assimilatory Inhibition: Journal of Linnean Society, 1896, pp. 385 
and 386. 
