Sunlight upon Aquatic Plants . 397 
piration keeps thin exposed leaves cool so long as an 
adequate supply of water is available, but in many fleshy 
leaves this is not the case, and the effect produced upon 
the power of C 0 2 -assimilation may be largely a thermal one, 
for the temperature of such leaves, when isolated, may amount 
to as much as 50° C. ( 1 . c., p. 444). When at the same time 
any discoloration is produced in the chlorophyll-bodies, the 
leaves, instead of being blanched, turn brown or yellow 
( 1 . c., Vanilla , Hoya , Cocculus Beccarii ), and it is curious that 
the action of light, combined with low temperatures, may 
cause a similar browning b Thus at ordinary temperatures 
the photo-chemical action of light causes a blanching of the 
chloroplastid, whereas at extremes of temperature a browning 
may be produced, if the leaves remain living for a sufficient 
length of time under such exposure. In the majority of cases, 
however, healthy and normal leaves are comparatively resis- 
- tant to prolonged exposure to direct perpendicular sunlight, 
even when the under surface is also illuminated, no perceptible 
effect being produced, or only a slight one in the surface 
layers immediately exposed. Often, however, in such 
apparently normal leaves, the assimilatory powers may be 
markedly affected, in parts or entirely. It is important 
to remember that in a condition of nature, so long as the 
temperature remains normal, it is very rarely indeed that 
the exposure is sufficiently intense or prolonged to produce 
any of the results which may be obtained under the much 
severer experimental conditions. As soon, however, as precise 
experimental data are obtained, it is easy to determine the 
amount of external exposure which any given plant can 
withstand in a condition of nature, knowing the conditions 
under which it exists and the protective adaptations which 
it possesses. 
1 Winter browning of Conifers, &c., Journ. of Linn. Soc., Vol. xxxi, p. 390. 
Botanical Laboratory, Oxford. 
