Ewart. — ■ The Effects of Tropical Insolation. 463 
Alps on clear nights, Linum is unable to empty its leaves 
of starch, whilst Satureja does so, and hence concludes that 
this is due to the red leaves being warmer during the night, 
an assumption which is not justifiable without direct experi- 
mental evidence, for it has yet to be proved that the red 
pigment has any special power of absorbing dark heat-rays : 
moreover, in correlation with the fact that the red leaf absorbs 
heat in slightly greater amount than a green one does, it 
would be natural to expect that the former would also radiate 
heat more rapidly than the latter when the temperature falls 
during the night. The above observations cannot be said to 
point one way or the other. All they show is that Satureja 
is more adaptable to an Alpine habitat, and with this 
adaptability the presence and increased formation of the 
red dye may be a more or less important correlative factor ; 
whilst Linum , under such conditions, is unhealthy, and 
amongst other signs of sickness a weakening of the trans- 
locatory powers may be one symptom. Costerus 1 has, how- 
ever, shown that, in most tropical plants, but little starch 
is removed during the night ; so that in the tropics, at any 
rate, the retention of an abundance of starch in the leaves 
over night is a normal phenomenon, and does not indicate 
that the plant is in an unhealthy condition. 
Stahl supposes that in the tropics the heat-absorbing 
function of the red pigment is to increase the amount of 
transpiration, and in the plants of temperate regions to raise 
the temperature of and hence give rise to more active 
metabolism in, more especially, the young growing organs. 
He points out that in the tropics many plants affecting 
shady habitats have a marked and deep red colouration, and 
that in many cases the red dye is present only in the under 
epidermis or spongy mesophyll. The explanation of the 
presence of a marked red colouration in the leaves of many 
shade-plants is readily explained as being necessary to guard 
against the partial or complete assim.ilatory inhibition to 
Costerus, Ann. d. Jard. Bot. de Buitenzorg, p. 73, Vol. xii, Pt. I, 1894. 
