132 
JOHN GRAY AND GEORGE J. PEIRCE 
tain the conditions of opening and closing and the connection of these phe- 
nomena with transpiration, a process the importance of which is inversely 
proportioned to the amounts of moisture in soil and air. Thus in the far 
west, where ''dry farming" is resorted to, it is of the utmost importance to 
the plant to be able to control its transpiration; and, at the times of most 
rapid evaporation from all wet masses, to be able to hold its water in spite 
of the temperature and dryness of the air. It is obvious that water vapor 
will escape more rapidly along a continuous volume of air, for example 
through an open stoma, than across one or more membranes, no matter 
how permeable, interposed between two volumes of air of different humid- 
ities, areas and other things being equal. What then causes the opening 
and closing of stomata? And what are the consequences? On these 
questions the studies of Darwin and Lloyd bear directly. 
Darwin (2), working under the well known climatic conditions of Cam- 
bridge, England, which are the general conditions in which the plant phys- 
iology of today has been developed, and Lloyd (3), experimenting at the 
Desert Botanical Laboratory of the Carnegie Institution, at Tucson, 
Arizona, with climatic conditions not altogether easy for the plant physiolo- 
gist of other so-called temperate climates to grasp, came to conclusions suffi- 
ciently divergent to justify us in reexamining the subject, especially as we 
live in a region the climatic conditions of which are near the mean between 
these two. Darwin, studying the leaves of ivy (Hedera helix L.) and ol 
laurel {Prunus laurocerasus L.), claims that transpiration is regulated 
mainly by the stomatal aperture. His results on individual leaves do not 
clearly show this; but examining the series as a whole, it is seen that the 
transpiration and the condition of the stomata appear to be related, usually 
with the relative transpiration somewhat exceeding the stomatal ratio. 
Lloyd worked on the ocotillo {Fouquieria splendens Engelm.) and a showy 
verbena {Verbena ciliata Benth.), both desert plants growing near his 
laboratory. The study of these extreme types led him to conclude that 
the ''regulatory function of the stomata is almost niiy During the day the 
outgo of water in the ocotillo is greater than the intake. The reverse is 
true at night. The stomata are open during the day, in which time also 
there is the greatest decrease in leaf weight. At no time is this effectively 
regulated by the movements of the guard cells. The stomata do not appear, 
therefore, to afford the plant any protection against evaporation during the 
times when evaporation is most dangerous. 
The opening and closing of the stomata have been attributed to the 
changes in the turgidity of the guard cells, their particular shape and their 
relations to the adjoining epidermal cells causing the changes in turgor to 
open and close the stomata. Von Mohl (4) and Schwendener (5) were the 
first to demonstrate this. The latter showed that the convex walls of the 
two guard cells, facing each other, are more extensible than the flat or con- 
cave walls adjoining the next cells of the epidermis. Increasing turgidity 
