Ewart . — The Effects of Tropical Insolation . 441 
shape and position of the palisade-parenchyma layers, and in 
the thickness, shape, and transparency of the epidermis and 
cuticle, adding to the protection afforded by the apostrophic 
position assumed by the chlorophyll-grains of the palisade- 
parenchyma when the illumination is intense. 
Another important mode of protection is by the folding or 
sinking of the leaves or leaflets when exposed to strong illumi- 
nation, so that their surfaces are parallel to the incident ray. 
In leaflets which fold together and overlap one another, and 
perhaps also to a slight extent in leaves or leaflets which 
simply twist on their axes, as a secondary consequence of 
the change of position, the amount of transpiration is more 
or less markedly diminished, and it is quite possible that in 
certain cases this may be an important function of the 
paraheliotropic movement. 
Finally, the presence . of a red pigment in the leaves of 
many plants must shield them to a certain extent from the 
effects of too intense and prolonged illumination. 
It must be remembered that the photochemical intensity 
of the tropical sun is by no means so great as might be 
supposed, and this is especially the case in moist, damp 
climates, such as those of Buitenzorg (W. Java) and Pera- 
deniya (S. W. Ceylon), in which the moisture causes an 
absorption more especially of the photochemical rays. In 
a recent research Wiesner 1 has shown that the greatest 
chemical light-intensity observed at Vienna was 1-5 units, and 
at Buitenzorg not more than 1-612 units, whilst the daily total 
amount in Buitenzorg during November and December is 
equal to that in August in Vienna, and the total amount 
daily in January at Buitenzorg is equal to that in June in 
Vienna, the greater intensity of the tropical light being 
counterbalanced by the shorter day. 
In several more or less shade-loving tropical plants, how- 
ever, prolonged exposure to perpendicular illumination 2 pro- 
1 Wiesner, Untersuchung iiber das Photochemische Klima von Wien, Buitenzorg, 
und Kairo : K. Akad. d. Wiss. in Wien, July, 1896. 
2 The necessary daily periods of uninterrupted sunlight were obtained in Java 
