98 STOMATA 



the latter are all placed parallel to the axis, as is usual for the 

 elongated leaves of Monocotyledons (Fig. 44, C) and the epidermis 

 of young stems ; in the latter case, however, the stomata are often 

 few in number (Fig. 44, D). Those of horizontal dorsiventral 

 leaves (e.g. of most Dicotyledons) are chiefly situated on the lower 

 surface, being sometimes altogether absent from the upper (e.g. 

 most trees), and, moreover, exhibit an irregular arrangement 

 (Fig. 44, B). In some families the guard-cells are accompanied 

 by so-called subsidiary cells (two in the Madder, Fig. 46, F, s.c., 

 and Bedstraws, three or more in the House-leek and Stone-crop, 

 Fig. 46, G, s.c.) which differ in size and shape from the other 

 epidermal cells, and probably form part of the mechanism of 

 the stoma. 



The stomatal apparatus originates from a mother-cell which 

 is cut off by means of a curved septum from one of the ordinary 

 epidermal cells. Soon a vertical wall is formed parallel to the 

 long axis of the mother-cell, separating the future guard-cells, 

 which gradually acquire their distinctive thickening. Mean- 

 while the middle lamella between them breaks down, except at 

 the two ends, to form the pore. Subsidiary cells, when present, 

 are usually cut off from the mother-cell before the guard-cells 

 are produced, but in some cases they are formed by division of 

 the surrounding epidermis. 



On mounting a strip of the fresh epidermis of some leaf in 

 water, the open pores of the stomata are readily visible, being 

 usually occupied by air. On transference to glycerine or a 5 

 per cent, sugar solution, which will reduce the turgor of the 

 guard-cells, the width of the pores decreases appreciably (i.e. 

 they " close "), but when returned to water the turgor is restored 

 and they again open. Measurement shows that, though there 

 is usually no marked change in length, there is an appreciable 

 increase in width, when the stoma opens (cf. Fig. 47). The 

 alteration in form of the guard-cells, to which the variation in 

 the size of the stomatal aperture is due, is thus primarily deter- 

 mined by changes in turgescence. 



The mechanism is, however, directly dependent on the un- 

 equal distribution of thickening which, in transverse section, 

 has been seen to be mainly localised around the upper and lower 

 edges of the pore ; on the other hand, the middle of the convex 



