332 Copeland . — The Mechanism of Stomata. 
increasing with their turgescence, cannot be guarded against 
by a hinge, for the latter must permit movement in one 
direction as well as in the other ; this is true even in the 
possible case that the wall acts as a spring, because any 
movement is from a temporary state of equilibrium, disturb- 
able in either way, else the motility of the stoma is interfered 
with. In many leaves, chiefly succulents, the stomata would 
be subjected to much greater, and probably irresistible 
pressure resulting from shrinking of the entire leaves with 
loss of turgescence ; but this pressure would be exerted 
against the guard-cells by the walls, and they (the guard- 
cells) are protected against it by thin bands 1 . Without such 
protection the stomata would be forced to shut, as probably 
occurs in the instance of Chrysoma 2 3 , whose cross-section 
shows only a hinge. 
In stomata whose outline changes with their movements, 
and only in these, the turgescence of the contiguous cells 
must be a factor in determining the state of equilibrium, open, 
closed, or intermediate. But because the pore closes with 
excessive transpiration when turgescence in the leaf is least, 
because the contents of the guard-cells furnish a clue to 
changes in turgor which is wanting in the neighbouring cells, 
because some stomata do not change their outline (surface 
view) in their movements, because isolated stomata usually 
move like those on uninjured leaves, and because the forms 
and structures of the guard-cells are explicable and intelligible 
on this ground only, the conclusion cannot be escaped that 
the turgescence of the neighbouring cells is a passive factor, 
the active one being, as Schwendener and his students have 
maintained, the turgescence of the guard-cells. 
We are now ready to turn to a study of the structural 
devices by which an increase in the turgescence of the guard- 
cells accomplishes the opening of the pore. The types 
1 W. Beneeke, Die Nebenzellen der Spaltoffnungen. Bot. Zeit. (1892), 521, 537, 
553, 569, 585, 601. 
3 F. E. Lloyd, Some points in the anatomy of Chrysoma pauciflosculosa. Bull. 
Torr. Bot. Club., xxviii (1901), 445-450. 
