MECHANISM OF STOMATA 



99 



wall next to the pore, as well as the curved wall remote from 

 the pore, remain comparatively thin (Fig. 46, E). When the 

 guard-cells are turgid and the thin walls stretch, the pull exerted 

 separates the thick walls, thus opening the pore (Fig. 47, op.). 

 As seen in section, opening of the stoma is accompanied by a 

 flattening of the convex walls on each side of the pore and an 

 increased convexity of the unthickened part. As a result there 

 is movement of the guard-cells at the lines of junction with the 

 adjacent epidermis both above and below, due to the bulging 

 of the walls away from the pore. 

 Where both the outer and the inner 

 walls of the epidermal cells are 

 thickened, there is a sudden thinning 

 at the points of junction with the 

 guard-cells to admit of their move- 

 ment (e.g. Onion, Fig. 46, E) ; but 

 when the external wall alone is 

 thickened, such a hinge occurs only 

 on the outside (e.g. Iris, Fig. 46, D). 

 The flattening of the convex walls 

 adjacent to the pore is facilitated 

 by the thinner middle portion, which 

 constitutes a similar hinge. A good 

 illustration of the action of a stoma, 

 depending likewise on unequal dis- 

 tribution of thickening, is afforded 

 by the change in form of the two 

 halves of a split Daffodil-scape, tied 

 -together at their ends with the outer sides in contact, and 

 placed alternately in salt solution and water. 1 



The influence of external conditions on the size of the sto- 

 matal pore can be indirectly studied by means of the porometer, 2 

 or by the following method adopted by Lloyd : Leaves, growing 

 in the particular conditions to be studied, are detached from 

 the plant and immediately placed in strong alcohol, which fixes 

 the guard-cells in their momentary condition. A strip of epi- 

 dermis can now be removed without alteration, and the average 



1 Cf. F. & S., p. 117. * 



2 F. & S., p. 369. 



FIG. 47. Half of a stoma, 

 seen in perspective, show- 

 ing the positions of the 

 guard-cells in the open 

 (op., firm lines) and closed 

 (cl., dotted lines) con- 

 ditions, ep., epidermis ; 

 g., guard-cell ; p., pore. 



