370 Curtis: SOME OBSERVATIONS ON TRANSPIRATION 
actual observation under the microscope but the rapid closing of 
the stoma in the case of plants standing in the shade suggests that 
possibly the balance between the tensions is so delicate that the 
least disturbance of the equilibrium, as in making the sections for 
examinations, may bring about the closing. It is also possible 
that, owing to the activity of the secreting cells and the accumu- 
lation of intracellular water, the stomata may open at varying 
periods and after the escape of the vapor close again. The rise 
in the rate after some one or more hours is perhaps due to the 
increase of water in the epidermal walls, induced by the turgor 
tensions. The work of Wiesner on the relative permeability of dry 
and moist membranes certainly points to such a conclusion. 
Comes has also demonstrated that the amount of water transpired 
from the two surfaces of the leaf when acting separately may be so 
increased that the total amount given off may exceed the normal 
transpiration of the leaf. The relatively large amount of water 
transpired in the third example after the sealing may be due to 
the quicker and stronger response in turgor tensions, thus bringing 
about a more rapid saturation of the epidermal membrane and 
there may also be an interaction on the stomatal apparatus. 
Another feature of transpiration that emphasizes the intimate 
relation existing between the process and the vital action of the 
plant is the periodicity to be seen in the opening and closing of 
the stomata. Darwin has recently shown that the stomata have 
acquired the habit of opening more readily in the morning under 
the influence of light than in the afternoon. This is very strik- 
ingly shown in the experiments that are recorded in table VI. 
The plants were given one hour illumination at various hours 
in the forenoon, indicated by the larger type, and for the same 
length of time in the afternoon. In the two last measurements 
the electric light was used. I have not a sufficient number of 
experiments at hand to warrant a conclusion as to the hour 
when the greatest response may be obtained and, owing to the 
very uncertain quality of the light from day to day as well as the 
influence of after effects, this would be a difficult if not impossible 
task. The results show in a very striking manner that there is 4 
pronounced periodicity in the stomata of the plants under con- 
sideration. The stomata appear to open quicker and wider in the 
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