28 STOMATA. 



other substances. Waxy matter is occasionally found in the epi- 

 dermis, silica is met with in the integument of grasses and Equiseta, 

 and carbonate of lime in that of Chara. The colour of the epi- 

 dermis generally depends on that of the subjacent parenchymatous 

 cells, from which it can be separated as a colourless layer. The 

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

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



Stomata (dTofia, 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. 

 72 s s, 74 s), supposed to resemble the lips and the orifice of the 

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

 or dryness in the atmosphere, — these changes depending on the 

 hygroscopic character of the cells. By examining, under the micro- 

 scope, thin stripes of epidermis in a moist and dry state, it wiU 

 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 collapse, approach each other, and 

 close the orifice. 



The cells surrounding the openings of stomata are sometimes 

 numerous, as in Marchantia. In. Ceratopteris thalictroides, Allman 

 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 {■m^i, around). 

 In Ficus elastica four cells form the stoma. In Equisetum, the 

 stomata, which are about tJtt of an inch in their greatest 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). Occasionally 

 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. 



Stomata communicate with intercellular spaces (figs. 76 s, 77 s), the 

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

 gation inwards of the cuticle, called, by Gasparrini, a cistoma {xiarrj, a 

 cyst or bag, and ero/iix, a mouth). They are scattered over the surface 

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

 regular iotervals corresponding to the union of the epidermal cells 

 (fig. 72 s) ; at other times they are scattered without any apparent 

 order (figs. 74, 75) ; 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. 78 s s), Orassula, and some Proteacese. 



Stomata occur on the green parts of plants, especially on the leaves 



