STOMATA. 25 



which communicate with the interior of the cell by canals passing 

 through the thickening layers, as in Cycas. In Eochea falcata (fig. 

 73), the epidermis, e e, consists of two layers of cells the outer ones 

 large, the inner small. The cells of epidermis are usually filled with 

 colourless fluid, but they sometimes contain resinous and other sub- 

 stances. Waxy matter is occasionally found in the epidermis, silica is 

 met with in the integument of grasses and Equisetums, and carbonate 

 of lime in that of Chara. The colour of the epidermis generally 

 depends on that of the subjacent parenchymatous cells. The epider- 

 mal cells are usually larger than those of the tissue below them; but 

 sometimes, for instance in Ficus elastica, they are smaller. 



50. Stomata (a-ropa, a mouth) are openings existing between some 

 of the cells of the epidermis on parts exposed to the air. They consist 

 usually of two semilunar cells surrounding an oval slit or orifice (figs. 

 69 s s, 71 s), which have been considered as resembling the lips and the 

 orifice of the mouth. Stomata open or close according to the state 

 of moisture or dryness in the atmosphere. By examining, under the 

 microscope, thin strips of epidermis in a moist and dry state, it will 

 be seen that in the former case the lips are distended, they assume a 

 crescentic or arched form, and leave a marked opening between them ; 

 while in the latter, they approach each other, and close the orifice. 



51. The cells surrounding the openings of stomata are sometimes more 

 numerous, as in Marchantia. In Ceratopteris thalictroides, Alhnan 

 observed stomata formed by three cells ; two of which, in their open 

 condition, are crescentic and concave inside, while the third surrounds 

 them, except at a small space at the end of the long axis of the stoma, 

 and has on this account been called peristomatic (vt^l, around). In 

 Equisetum, the stomata, which are about 5 | 5 of an inch in their great- 

 est diameter, consist of four pieces ; two of which are arched and thick 

 at their outer convex margin, becoming thin at their inner concave 

 edge, where two other bodies occur, having numerous processes like 

 the teeth of a comb, hence called pectinate (pecten, a comb). Occa- 

 sionally the stomatic cells become united, so as to appear in the form 

 of an uninterrupted rim; and at other times the stoma is a minute 

 orifice in the walls of a cavity. 



52. Stomata communicate with intercellular spaces (figs. 73 s, 74 s), 

 the connection being sometimes kept up by means of a funnel-shaped 

 prolongation of the cuticle, called, by Gasparrini, a cistoma (wry, a 

 cyst or bag, and trrof^a,, a mouth). They are scattered over the surface 

 of the epidermis in a variable manner. Sometimes they are placed at 

 regular intervals corresponding to the union of the epidermal cells 

 (fig. 69 s); at other times they are scattered without any apparent 

 order (figs. 71, 72) ; and in other instances they are united in sets of two 

 or three, or in clusters at particular points, as may be seen in Begonia, 

 Saxifraga (fig. 75 s s), and Proteacese. 



