OUTER CELLS AND TISSUES. 



27 



niesophyll cells of a leaf, or cortical cells of a stem ; they also 

 allow of the exit of aqueous vapour during transpiration, a most 

 important function, and also of oxygen during assimilation. 



A single stoma arises by the division of a young epidermal cell 

 into two, and these separate slightly along the line of junction 

 known as the middle lamella, leaving an opening which leads 

 into the afore-mentioned space (see Fig. 16, a). The walls of 

 these cells become greatly thickened, but the cell-contents persist; 

 and a certain amount of apparent subsidence may take place, as 

 in Pinus. so that ultimately the cells, which are known guard- 

 cells, come to lie somewhat below the general level of the 

 epidermis (see Fig. 15, h). 



Fig. 12. — A Stoma from the Leaf of Smilax, seen in surface view. Tlif- 

 two crescentic guard-cells possess cytoplasmic contents and ohloroplasbs. 



Fig. 13.^ — A Stoma from the Leaf of Iris, in surface view (from a, 

 photomicrograph). 



The primary cells may divide more than once, the last division 

 of all resulting in the formation of guard-cells ; the first-formed 

 cells are then termed " subsidiary." Subsidiary cells are well 

 seen in the leaf of Sedum (see Figs. 12 to 14). In surface view 

 guard-cells are usually crescentic in shape. 



A section across a stoma will show the following features : — 



i. An outer passage, the "vestibule," bounded, as a rule, by epidermal 

 cells, or at times by subsidiary cells. The guard-cells lie at the inner end 

 of the vestibule, and are very close together, leaving only a very narrow- 

 entrance into 



