THE ANGIOSPERMAE : LEAVES 



lOI 



producing a stomatal mother cell surrounded by a variable number of 

 subsidiary cells. The majority of families, however, produce subsidiary 

 cells, if at all, by sub-division of the neighbouring epidermal cells and the 

 initial cell divides only once to form the two guard cells of the stoma. 

 Between these the pore appears, owing to the splitting of the wall which 

 separates them. There is much variation in the details of stoma formation, 

 many of which are peculiar to certain families and can be used as systematic 

 characters. 



When first formed the guard cells equal the epidermal cells in depth, 

 and lie in the same general plane. The sunken position of many stomata 



1003. — Z.ea mais. Structure of the stoma. A, Surface \ic\v. B, Longitudinal section 

 of guard cell. C, Transverse section through the ends of the guard cells. D, Transverse 

 section through the stomatal pore and middle of the guard cells. E, Surface view, 

 guard cells in open and closed positions. {After Cope/and.) 



is due to later growth of the epidermis and sometimes to its periclinal division 

 into two or more layers. 



The shape of the guard cells is remarkably uniform, though thev var\' 

 considerably in size. They are curved in two directions. Firstly, there is the 

 obvious concavo-convex curvature which separates them from each other, 

 except at their ends. This curvature is variable and affords the means of 

 controlling the stomatal opening. Secondly, there is a curvature less easily 

 seen, perpendicular to the surface, the extremities of the cells being turned 

 somewhat downwards. This curvature does not vary as the first does. The 

 upper and low^er walls are highly thickened, while the lateral walls are relatively 

 thin. 



The Gramineae are remarkable for guard cells of a peculiar pattern (Fig. 

 1003). They are straight, not curved, and shaped like a pair of dumb-bells, 

 side by side. The middle portions are highly thickened, while the expanded 



