462 Ewart . — -The Effects of Tropical Insolation , 
the protoplasm. Were the primary and most important 
function of the red pigment to act as a heat-absorber, it 
would be natural to expect that it should show a marked 
absorption of the dark heat-rays which do not appear to 
be used in chlorophyllous assimilation, instead of its absorb- 
ing for the most part the green and yellow rays, in which 
the heating effect is comparatively slight. Engelmann (1. c.) 
does not, however, ascribe any power of absorbing the dark 
heat-rays to erythrophyll. Stahl (1. c.), it is true, finds that 
red leaves, when exposed to dark heat-rays, show a rise 
of temperature of from i° to 2° over similar green leaves 
similarly exposed. The explanation of this probably lies, 
however, in another observation of Stahl’s, namely, that red 
leaves have fewer stomata than the green ones, the conse- 
quently lessened transpiration in the red leaf being probably 
the cause of the difference in temperature. It is extremely 
probable that in the water distributed throughout its sub- 
stance, the leaf possesses as sufficiently powerful an absorbent 
of the dark heat-rays as is needful. The very fact that in 
spotted leaves the red areas contain fewer stomata than do 
the neighbouring green ones, is very strongly in favour of the 
view that the slight additional absorption of heat and con- 
sequent acceleration of transpiration is a disadvantage which 
needs to be guarded against, and which is in this manner 
adequately compensated for. As Stahl shows, green leaves 
or green areas of the leaf, when exposed to sunlight in dry air, 
give off more water-vapour than red leaves of the same plant 
or red areas of the same leaf do. 
Kerner von Marilaun 1 mentions that Satureja hortensis, 
grown fully exposed to sunlight in his alpine garden, develops 
an even greater abundance of the protective red dye 
and flourishes, whilst Linum usitatissimum remains green, 
grows badly, and dies before flowering. From this he con- 
cludes that the red pigment is important as a protection 
against intense illumination. Stahl (1. c.) finds that, in the 
Pflanzenleben, Bd. i, p. 364. 
