i6 THE STRUCTURE OF ECONOMIC PLANTS 



in 



B 



vacuole. Chloroplasts are commonly absent except in specialized 

 guard cells. The walls may be unequally thickened, the outer one 

 most cases being the thickest, while the radial walls are fre- 

 quently thicker than the inner one. 



The Stomata. — The distribution and num- 

 ber of stomata vary with the species, and also 

 with the organ of the plant. In general, they 

 are more abundant on leaves than on stems; 

 and, in the former, more numerous on the 

 adaxial surface than on the abaxial; although, 

 in some instances, as in the grasses, they occur 

 in approximately equal frequency on the two 

 surfaces. The stoma is a slit-like opening in 

 the epidermal layer which permits movement 

 of gases between the outer atmosphere and the 

 intercellular spaces of the mesophyll. It is 

 bounded by a pair of guard cells with concave 

 facing surfaces and contiguous end walls; and 

 the size and shape of the stomatal aperture is 

 determined by the guard cells, and accessory cells 

 when present, varying with changes in their 

 turgidity. The guard cells may have charac- 

 teristic wall thickenings that are correlated 

 with their turgor movements. In some cases, 

 the walls are uniformly thickened and the 

 Fig. 5. /I, surface view guard Cell appears round or elliptical in tran- 

 of the stoma of wheat section; but, more frequently, each guard cell 



showing guard cells and , n 1 ■ 1 • r 



accessory cells; B, median has two parallel ridges on its concave surface 



transection of same; c, which form protruding flange-like margins 



S^rnglrcrrj^fi Ordering the outer and inneV limits of the 



cell. (Redrawn from Per- aperture. The outcr ridge is generally more 



civai. m Wheat Plant, pj-Q^^inent than the inner, which may be 



Duckworth and Co.; n 



lacking. (Fig. 5, 5.) 

 The position of the guard cells in relation to the outer surface 

 of the epidermis is variable. In Allium, the level of the guard 

 cells does not equal that of the epidermis; and they are somewhat 

 depressed, forming a cavity which may be overarched by the 

 adjacent epidermal cells. In Ipomoea, Solanum, and Zea the 

 guard cells, though smaller than the epidermal cells, are so oriented 

 that they project slightly above the epidermal surface; while 



