INFLUENCE OF LIGHT UPON ACTION OF STOMATA 
stretches the guard cells, their walls flatten or remain unchanged according 
to their thickness, and the slit between the two opens and widens. De- 
creasing turgidity has the opposite effect. Thus the direct connection 
between turgor changes and stomatal movements has been generally ac- 
cepted, and Darwin and Loyd seem to assume, in their investigations, that 
this is the case. 
Our study of these papers has led us to make certain experiments which 
cause us to conclude first, that the turgidity of the guard cells is a necessary 
factor in producing and maintaining their elasticity, but second, that the 
direct and indispensable agent in controlling the opening and closing of the 
stomata is sunlight, which acts as a stimulus upon the guard cells themselves. 
Their peculiar form and position give them a freedom of movement, in con- 
sequence of differences in turgidity, quite unlike that possessed by any other 
cells of the body of land plants. 
Before any reasonable comparison of the results of Darwin, Lloyd, and 
ourselves may be made we should note the conditions under which the par- 
ticular plants studied were grown, as well as the methods used in securing 
the different results. The conditions of the three sets of experiments 
present two extremes and a favorable mean. The ivy and laurel which 
Darwin used were growing in the moist climate of eastern England: the 
ocotillo and verbena of Lloyd are typical xerophytes of the Arizona desert. 
Ocotillo, or "coach-whip cactus," is lithe, slender, unbranched, and in 
appearance and structure like the cactus among which it grows. Verbena 
is a hardy perennial, persisting through the annual period of drought by 
means of its deep roots and withering stems. Intermediate between the 
extremes of England and Arizona are the climatic conditions at Stanford 
University, California (6), where the humidity is fairly high at all times, 
and where the temperature, while varying considerably, very rarely reaches 
an extreme of heat or cold. At times the vegetation is subjected to con- 
ditions as moist as those of England. This is true of certain rainy seasons 
particularly, and we find here a rainy season flora, typified by Montia per- 
foliata (Donn.) Howell, very different from that of the dry season, of which 
the Hemizonias may be taken as characteristic. 
It is well known that plants develop more or less different structural 
characters in different environments. Where the humidity is highest, the 
cuticular covering of the epidermis is thinnest, other things being equal; 
and where, as in the deserts of Arizona, the humidity is low and the tem- 
perature high, the cuticula is thickest. It is important to bear this in mind, 
since it is obviously possible that, in certain circumstances, cuticular trans- 
piration or evaporation may take place. While it is true that the cuticula 
protects the underlying tissues from evaporation, it does so only incom- 
pletely. Stahl's cobalt chloride test (7), which is purely qualitative, is not 
delicate enough to demonstrate cuticular evaporation, or cuticular trans- 
piration: but the method of Buscalioni and Polacci (8) may be employed 
