Ewart — The Effects of Tropical Insolation. 475 
phyll which they contain enables them to dispense with any 
special and very much less efficient heat-absorbing medium 
such as erythrophyll. 
That the stomata are markedly sensitive to and may be 
injured by prolonged exposure to sunlight, so that the 
presence of a protective red dye in the guard-cells might 
possibly under given conditions of exposure be an advantage 
to the plant, is shown by the following observations. The 
adult leaves of a number of plants having stomata on both 
surfaces were exposed to bright sunlight, in some cases the 
upper and in others the under surface being the exposed one : 
the epidermis was then removed and examined under optimal 
illumination by the Bacterium-method to ascertain whether 
the assimilatory powers of the chlorophyllous guard-cells 
were affected in any way. The guard-cells are rather sensi- 
tive to the action of prolonged sunlight ; and often four to 
five hours’ exposure, especially when it is the under surface 
which is exposed, is sufficient to inhibit their assimilatory 
activity, the power as a general rule returning again in from 
a few hours to a day, if placed under conditions which permit 
of recovery. With longer exposure, and especially if sub- 
jected to the cumulative effects of exposure extending over 
several days, the chlorophyll-grains are more or less com- 
pletely bleached, and may be in a condition of permanent 
light-rigor from which no recovery is* possible. It is 
commonly the case that in exposed leaves which have been 
for some time adult but which are still quite vigorous, no 
power of assimilation can be detected in the guard-cells 
of the stomata, if any, on the upper surface ; and this may 
still be the case, especially in well-exposed leaves, even if 
previously kept under conditions which permit of recovery, 
the chloroplastids in such cases being in a condition 
of permanent light-rigor. The guard-cells from the under 
surface of normal leaves, when examined in the morning, 
almost without exception show a more or less marked power 
of assimilation, and this persists until the decline of the leaf 
K k 
commences. 
