476 Ewart . — The Effects of Tropical Insolation. 
Bearing these facts in mind, it is easy to understand that, 
in much exposed and horizontally expanded bifacial leaves, 
it should be an advantage to the plant to have the stomata 
restricted entirely or almost entirely to the under surface 
of the leaf ; whereas in an erect leaf, or one growing in the 
shade, the stomata tend to be more evenly distributed over 
both surfaces. It is interesting to notice that in a bifacial 
leaf, with more than one layer of palisade-parenchyma, 
owing to the stomata being more abundant on the under 
surface of the leaf, the amount of assimilation performed by 
the different layers of the palisade-parenchyma tends to be 
equalized under normal conditions of exposure and illumina- 
tion, the deeper layers coming into contact with air containing 
a higher percentage of C0 2 ] , whilst the upper ones are 
exposed to stronger illumination. 
Stahl mentions that the marked red colouration which the 
stigmas of many anemophilous plants show, may be for 
the purpose of raising the temperature of the stigma, and 
hence aiding the growth and germination of the pollen, on 
which, according to Stahl, light exercises no perceptible 
influence. In a previous paper 2 it has however been shown 
that exposure to light of marked intensity exercises a distinct 
influence on the germination of the pollen-grain, and more 
especially retards the growth in length of the pollen-tube. 
Hence, considering the generally much exposed condition 
of the stigmas of anemophilous plants, it is very evident 
that the red dye even in this case may be of paramount 
importance as a protection against the inhibitory action 
which strong light exercises on the growth and elongation 
of the pollen-tube. 
In the case of alpine and arctic plants, the slight additional 
absorption of heat which the red dye causes, may be of some 
use ; but in all cases, other things being equal, an increased 
absorption of heat means an increased transpiration, which, 
1 Blackman, On the Path of the Gaseous Interchanges in the Leaf : Proceedings 
of the Royal Society, 1896. 
2 Observations on the Pollen-tube: Trans. Liverpool Biol. Soc., Vol. ix, 1895. 
