Ewart. — The Effects of Tropical Insolation . 461 
M.pudica , and agitates any part of it at all strongly, the green 
appearance disappears at once, and only an apparently withered 
clump, in which the hard and prickly stems are most con- 
spicuous, remains ; the consequence being that the animal either 
turns away or passes through the clump of Mimosa to reach 
more congenial and less bewildering pasturage. In a country 
with many browsing animals this peculiarity cannot fail to 
be of considerable, though only accidental, importance. 
As regards the second part of Stahl’s theory, there is no 
doubt whatever that the presence of erythrophyll in the 
cell-sap will cause a leaf to absorb more light, and also more 
heat, than it would otherwise have done ; but the question is 
whether the heat-absorbing property of the red dye is not 
merely secondary, and, perhaps, in many cases, a disad- 
vantage to the plant. In the case of alpine plants, Kernel* von 
Marilaun 1 had already pointed out that the heat-absorbing 
power of the red dye may be of accessory importance to its 
light-protective function. 
Engelmann 2 has shown that the red pigment erythrophyll 
allows, in a typical case, about 90 per cent, of the orange 
rays which are most useful for assimilation, 10 to 30 per cent, 
of the green and yellow which are least useful, 50 per cent, of 
the blue which are somewhat more useful, and 80 per cent, 
of the violet, rays to pass through. 
Pringsheim 3 found that when exposed to concentrated 
blue or green sunlight in the presence of oxygen, the 
chlorophyll-grains over an exposed area of an end-cell of 
Char a are killed and bleached in 5 minutes ; whereas when 
exposed to the concentrated red rays and kept cool, they 
remain living for a longer time, only beginning to bleach after 
20 minutes’ exposure. The red dye therefore, whilst allowing 
as much as possible of the light useful for assimilation to pass 
through, absorbs as much as possible of the rays exerting the 
greatest photo-chemical effect upon the chlorophyll and 
1 Pflanzenleben, Bd. i, p. 364. 
2 Engelmann, Bot. Zeit., June, 1887. 
3 Pringsheim, Pringsh. Jahrb., Bd. xii, 1882, pp. 326-344. 
I i 2 
