366 Curtis: SOME OBSERVATIONS ON TRANSPIRATION 
subject to variation. Saturation, tension of gas, salts, food con- 
tents, metabolic processes, conditions of growth, all, acting inde- 
pendently of outward conditions, have in a degree a controlling 
influence on the cell activity and the expression of this complex 
interaction is found in the fluctuations. It is not intended to imply 
that transpiration is a purely physiological phenomena. The 
position of Nageli is the only rational one—all physiological proc- 
esses are dependent upon chemical and physical laws and every 
vital phenomenon goes on in accordance with these laws. This 
statement of Nageli’s made in 1861 would almost seem prophetic 
in the light of the knowledge on this subject of vital phenomena 
held to-day. We are accustomed to look upon the phenomena of 
transpiration as a purely physical process, largely, no doubt, 
because it is so intimately associated with the factors that govern 
evaporation. However, in this consideration we overlook the 
fundamental character of the physiological processes involved in 
transpiration and that are of the highest importance in growth, 
development and gaseous exchange. And it may be pointed out 
that the manner of action of light upon transpiration is by no 
means a settled problem, aside from the fact that it is intimately 
associated with the activities of the cell. Certainly the behavior 
of plants in light and darkness and the reaction of etiolated, 
chlorophylless and green plants to light would be difficult to 
explain on a purely physical basis. Again the after effects attend- 
ing variations of illumination indicate interactions of a physiolog- 
ical character. If the vaporization of water in the plant is of a 
mechanical nature, having for its purpose the diffusion of the 
meager supply of salts required by the plants, then the process is 
not in keeping with the exceptional economy illustrated in the 
activities of the plant. It appears as rather remarkable when W 
consider the devices for the distribution of the slowly diffusing 
organized products, that the plant never hit upon an adaptation 
that would provide it with this relatively minute quantity of salts 
without putting it to the expenditure of so much energy. This 
supposition certainly ranks transpiration as perhaps the most 
wasteful operation in the vegetable kingdom. 
The most variable factor in the external conditions of the * 
periments mentioned above was the variation in the percentage © 
