474 Ewart . — The Effects of Tropical Insolation. 
simply due to the fact that the young foliage, which in most 
tropical plants is more or less tinged with red, is very much 
more abundantly formed at this period than during the dry 
season. Even during the wet season in West Java, there is 
almost always bright sunlight until mid-day, lasting often till 
3 or 4 p.m., and occasionally all day ; so that the young 
foliage which the rain has caused to be produced in such 
abundance is exposed for six hours on the average to 
very bright illumination, the sunlight from 9-12 being the 
brightest of the day. Hence the protective red colouration 
is perhaps quite as necessary during the wet season as during 
the dry. 
An interesting fact noticed by Stahl ( 1 . c.) is that in the 
adult leaf the guard-cells of the stomata are always without 
any red dye even though it may remain present in the 
epidermal cells of the upper surface. The importance of this 
Stahl concludes to be that the guard-cells remain cooler, 
transpire less, keep open longer, and hence promote trans- 
piration and assimilation. However, Stahl also points out 
that in many plants, especially in spring, the epidermal 
cells immediately around the stomata have a red sap, and 
are thereby enabled to heat the points at which the water- 
vapour escapes and hence the guard-cells also. 
Instead of adopting either of these contradictory explana- 
tions, it is perhaps simpler if we regard the absence of the 
protective red dye from the guard-cells of the stomata as 
being due to the fact that they are essentially organs which 
react to light : hence it is important that they should be 
exposed to the same intensity of light as is falling on the 
surface of the leaf in which they are present, even though 
a certain risk of permanent injury may thereby be incurred. 
Wiesner 1 states that a portion of the light absorbed by the 
chlorophyll goes to warm the chlorophyll-grain, and hence 
the surrounding parts also ; so that even if we accept Stahl’s 
view, the absence of the red dye from the guard-cells may 
simply be due to the fact that the greater amount of chloro- 
1 Wiesner, Sitzungsbericht d. K. Akad. der Wiss. zu Wien, Bd. Ixxiv. p. 1876.. 
