9 6 



STOMATA 



that this may possibly have in enabling the plant to place its 

 leaves at an appropriate angle to the light, 1 the concentration 

 of the latter may well be of value in connection with carbon 

 dioxide assimilation. 



The general characteristics of stomata are readily studied by 

 stripping off a piece of the epidermis from a fresh leaf of the Iris. 

 Scattered among the colourless elongated cells are oval stomata, 

 each consisting of two bean-shaped guard-cells (Fig. 46, C, g.c. ; 

 see also Fig. 44, C) surrounding the narrow elliptical pore by 

 means of which the intercellular spaces of the leaf communicate 

 with the atmosphere. The guard-cells contrast with the ordinary 

 epidermal cells in containing numerous chloroplasts and starch- 

 grains, and are especially distin- 

 guished by the uneven thickening 

 of their walls, which are thin on 

 the side away from the pore, but 

 markedly thickened around the latter 

 (Fig. 46, C). Each guard-cell has 

 a well-developed protoplast and a 



FIG. 45 .-Transverse section prominent nucleus. The ordinary 



epidermal cells almost meet above 

 the guard-cells, so that the latter 

 are only plainly visible on focussing 

 to a lower level (Fig. 46, C) ; in 

 this way the pore comes to be 

 situated at the bottom of a miniature 



hollow (vestibule) constituted by the surrounding cells, and the 

 stoma consequently occupies a sheltered position. This feature 

 is especially marked in plants of dry situations, but in those 

 growing in damp, shady habitats the guard-cells are often level 

 with or raised above the adjacent epidermis (Fig. 81, C, D). 



The overarching of the guard-cells by the adjoining epiderm; 

 cells is equally obvious in a transverse section (Fig. 46, D) of 

 the /ns-leaf. Here each guard-cell appears approximately ov; 

 in form and provided with thick walls, the thickening beinj 

 especially marked adjacent to the pore, and giving rise to 

 slight upwardly projecting ridge at the outer edge of the aperture. 

 In many plants, however, the walls of the guard-cells are not 

 1 Cf. F. & S., p. 212. 



through part of the upper 

 epidermis and palisade 

 layer of the leaf of the 

 Moschatel (A doxa moscha- 

 tellind). The chloroplasts 

 are shown black. 



