Copeland . — The Mechanism of Stomata. 345 
The Gymnospermae. 
From the work of Hildebrand 1 , Kraus 2 , Strasburger 3 , 
Mahlert 4 , and Klemm 5 , it has long been recognized that the 
stomata of all Gymnosperms constitute a well-defined morpho- 
logical type. They occur individually at the bottom of 
depressions, the wall just above them showing a more or less 
thin hinge. In median cross-section (Fig. 24) the lumen is 
shallow, and there can be distinguished, as Kraus says, only 
an outer thick and an inner thinner wall. Both walls are too 
thick to be bent — not to suggest stretched — by a pressure 
from the contents. They are strongly lignified and, where 
exposed to the air, cutinized. The part remaining cellulose 
varies in different genera. It does not look probable to me 
that there is any considerable movement of water through 
this wall (cf. Schwendener, 1881 , p. 836), the ends of the 
guard-cells affording a readier passage. A cross-section at 
the end of the stoma throws the needed light on the 
mechanism. The outer wall is narrow, and the dorsal wall 
inclined about 45 0 , making the cell almost a right-angled 
triangle in section, the dorsal wall being the hypotenuse. 
The walls are thin throughout, except for a part of the dorsal. 
When the guard-cell takes up water the dorsal wall will be 
forced in the direction indicated by the arrow in Fig. 25, 
carrying with it the thick-walled part of the cell and opening 
the pore. The thickened line running along the dorsal wall 
helps the active ends to move the passive middle zone, and so 
permits a longer rift. The very oblique polar walls and 
hinges already .mentioned facilitate the free movement of the 
guard-cells in the manner just described. 
1 F. Hildebrand, Der Bau der Coniferen-Spaltoffnungen und einige Bemerkungen 
iiber die Vertheilung derselben : Bot. Zeit. xviii (i860), 149- 15 2. 
2 G. Kraus, U. d. Bau d. Cycadeenfiedern : Jahrb. f. wiss. Bot. iv (1865), 305-346. 
3 E. Strasburger, Ein Beitrag zur Entwickelungsgeschichte der Spaltoffnungen : 
Jahrb. f. wiss. Bot. v (1866), 297-342. 
4 Mahlert, Beitrage zur Kenntniss der Laubblatter der Coniferen, mit besonderer 
Beriicksichtigung des Spaltoffnungsapparates : Bot. Centralb. xxiv (1885), 54, 85, &c. 
5 P. Klemm, Ueber den Bau der beblatterten Zweige der Cupressineen : Jahrb. 
f. wiss. Bot. xvii (1886), 498-540. I have been unable to see Striibing’s Konigs- 
berg thesis, of 1888, on this subject. 
