102 DEVELOPMENT OF A STOMA [CH. 



partition as in many Ferns. This cell becomes the initial 

 cell of the stoma. In many cases other cells are also cut 

 off around the mother cell and invest it as subsidiary 

 cells (e.g. Commelyna, Sedum, Pteris, &c.). 



The initial cell, which is richer in living contents than 

 the others, then becomes more or less regularly oval, and 

 a straight median partition- wall, also vertical to the 

 plane of the epidermis, is formed along its longer axis. 

 This wall then splits, by the dissolution of its middle 

 lamella, the process commencing in the centre and ad- 

 vancing to either end, and the aperture thus formed is 

 the pore of the stoma, bounded by the two guard-cells : 

 it leads directly into the simultaneously formed inter- 

 cellular space (sub-stomatal cavity) below, and thence to 

 the rest of the intercellular spaces of the mesophyll. 



As the pore forms, the chlorophyll-corpuscles of the 

 guard-cells attain their mature size and green colour, and 

 the outer cell- walls become thickened : the neighbouring 

 cells rarely develope the green chlorophyll-corpuscles. 

 The cuticle also developes, and extends over the lips of 

 the pore. Usually the floor, as well as the roof of the 

 guard-cells, is also thickened; while the walls next the 

 aperture and those adjoining the subsidiary cells or other 

 epidermal cells remain thin and extensible. 



The guard-cells thus formed are as a rule not only 

 smaller in area than the neighbouring cells, but they are 

 far less deep from roof to floor : it commonly results that 

 they are sunk beneath the general level of the epidermis, 

 though in rarer cases they project beyond the upper 

 margins of the neighbouring cells. 



Since the latter are almost invariably thickened and 

 provided with a firm cuticular layer at their roof, while 

 their floors and lateral walls in contact with the guard- 

 cells are thin and flexible, the guard-cells are suspended 



