96 



STOMATA 



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

 leaves at an appropriate angle to the light/ 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-shapecl guani-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 especialty distin- 

 guished by the uneven thickening 

 '^^'^r^ ~~'°'\=^ '^^ their walls, which are thin on 



_ /(^ ^^ ^ ^ -~n!'— ^'^'^ ^^'^'^ away from the pore, but 



markedly thickened around the latter 

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

 a well-developed protoplast and a 

 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 miiriature 

 hollow (veslibtile) 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 epidermal 

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

 the Iris-leai. Here each guard-cell appears appro.\imately oval 

 in form and provided with thick walls, the thickening being 

 especially marked adjacent to the pore, and giving rise to a 

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

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



' a. ¥. & s., p. 212. 



Fig. z|5. — Transverse section 

 through part of the upper 

 epidermis and paUsade 

 layer ot tlic leaf of the 

 IMoschatel [Adoxa moscha- 

 teU'nia). The chloroplasts 

 are shown black. 



