47 2 Eli' art.- — The Effects of Tropical Insolation. 
retardation of the growth and turning green of the chlorophyll- 
grains. If the leaflets are merely spread out horizontally, 
but not otherwise exposed, so that the rate of transpiration 
is markedly increased, they may become wilted, and their 
development and rate of growth is slightly retarded. This 
is hence certainly a case in which any power of increasing 
the rate of transpiration which the red dye may possess in 
virtue of its feeble action as a heat-absorbing medium, does 
not seem to be of any advantage but rather a slight dis- 
advantage to the plant, were it not guarded against by the 
hanging and overlapping position of the leaflets ; whilst the 
protection which the red dye affords against the retarding action 
that light of too great intensity exercises on the growth and de- 
velopment of the young and especially sensitive chlorophyll- 
grains is clearly of the highest importance. The young leaf is 
not to be regarded as an assimilatory organ, the metabolism 
of which demands a large supply of mineral salts and hence 
also of water, but as a growing organ to which nourishment 
is supplied in soluble and concentrated form, and in which 
a comparatively small amount of transpiration suffices, along 
with the strong osmotic currents maintained by the continual 
removal of the supplied food-material, to provide it with all 
it requires. 
If the primary function of the red dye in the tropics were 
to increase the amount of transpiration, then it would be only 
natural to expect that it would be formed in greatest 
abundance where the temperature is lower and the air more 
nearly saturated with water-vapour. The very opposite is 
however the case. Thus at the foot and sides of the volcanic 
mountain of Gedeh, and in the valleys around, very many 
plants have a reddish colour, especially in the young leaves. 
As one ascends this becomes less marked, until at Tjibodas 
and in the forests above it (4,500 ft. to 6,500 ft.), the number 
of plants showing a red colouration, and the intensity of the 
latter when present, reach a minimum. The vegetation at 
this elevation is almost entirely green, a few plants only, 
especially if growing in open clefts or glades in the forest, 
