330 Copeland . — The Mechanism of Stomata. 
phytes, and plants with nyctitropic leaves (Stahl 1 , Darwin). 
The stomata of such hydrophytes as have them must, in 
general, be regarded as at least slightly motile (Leitgeb, 
Haberland 2 , Darwin) ; Kohl and Schellenberg would regard 
most of them as entirely closing the pore. Stahl 3 is the 
most positive supporter of the view that even among 
terrestrial plants nocturnal closure is no general rule. For 
the study of transpiration, his method — testing for escaping 
water — is certainly better than direct microscopic examina- 
tion of the stomata. But when the stoma is itself the subject 
of study, direct observation is the only conclusive method ; 
and comparatively few stomata have as yet been seen open at 
night. We owe these largely to Leitgeb. I shall assume 
then that most stomata are motile, and that the pore is more 
or less completely closed in darkness. 
Greater precision than has hitherto been observed in the 
use of terms is prerequisite to an analysis of the mechanism 
of the movements. Turgescence and turgor have been used 
in this connexion indiscriminately for the osmotic pressure 
which the dissolved material in the cell-sap can exert, and for 
the condition of turgidity. Restricting turgescence to the 
latter sense, let us distinguish the former as turgor ; the word 
is already most familiar in that use — to indicate the osmotic 
power of the cell-sap, usually measured by plasmolysis. The 
turgor in the guard-cells must vary, being increased by light 4 
or heat. The turgescence depends upon the turgor and also 
upon the available supply of water. In direct sunlight and 
proper warmth, in spite of the increasing turgor, the evapora- 
tion of water may lessen the turgescence of the guard-cells, 
1 E. Stahl, Ueber den Pflanzenschlaf und verwandte Erscheinungen. Bot. Zeit., 
lv (1897), 71-109. 
3 G. Haberlandt, Zur Kenntniss des Spaltoffnungsapparates. Flora, 1xx(i887), 
97-109. 
3 E. Stahl, Einige Versuche liber Transpiration und Assimilation. Bot. Zeit., liii 
(189T, II7-I45- 
4 This assumed variation in turgor needs careful study : a difference in turgor by 
day and by night has never been measured. Nor are the osmotically active sub- 
stances known. Kohl tells us that the action of diastase opens the pore; but 
diastase, like sugar, is unknown in guard-cells. 
