176 



MR. H. HAMSHAW 



AND MISS N. BANCROFT ON 



and Ceratozamia mexicana,i\\aii the polar subsidiary cells represent ordinary epidermal 

 cells, while the lateral ones are the product of division of the epidermal mother-cell 



I'requently the lateral cells are transversely 



giving rise to the guard-cells also. 



divided into two, so that six subsidiary cells 



ivi 



polars and four laterals 



seen. Occasionally 



one of the lateral cells is divided; division of 



are 

 polar 



Fig. 26 



A. 



4 



B 



^ 



Bowenia sjjectabilis var. serrulaUt. — A, surface views of stomata {cf. PI. 17. figs. 7 & 8) ; 

 B, 1-4, serial sections through a stoma, corresponding to the positions 1-4 in the surface 

 view. These sections show the gradual raising of the guard-cells towards the polar ends 

 of the stoma, and the change in size and position of the dorsal {d) and ventral (v) lignlfied 

 lamellte of the guard-cells, these changes giving the characteristic surface view of the 

 stoma. B, o, shows a polar section of a stoma in which the dorsal lamellae do not appear: 

 this section corresponds to position 5, in the surface view. (Diagrammatic.) 



cells is comparatively infrequent. An increase in the number of subsidiary cells 

 is often seen where guard-cells are much depressed below the general level of the 

 epidermis, there being a mechanical necessity for this increase— the deeper the outer 

 air-space, the greater the number of cells required to ensure its rigidity, by means 

 of their lateral walls. 



