446 VENTILATING SYSTEM 



stomatic apparatus in the strict sense of the term. [The whole 

 structure, including the guard-cells and the orifice which they surround, 

 is termed a stoma.] Not infrequently some of the epidermal elements 

 immediately adjoining the guard-cells are also specialised as subsidiary 

 cells of the stoma. Those cells, finally, which border upon the inter- 

 cellular air-chamber that underlies the stoma, may also exhibit various 

 peculiarities of form or structure. In such cases, the definition of the 

 stomatic apparatus may be extended so as to include not only the 

 guard-cells, but also the subsidiary epidermal cells, as well as any 

 specially modified elements of the underlying tissue that may be 

 associated with the stoma. 



Every normal stoma has the power of expanding and contracting, 

 of opening widely and of closing [more or less] completely ; it is by 

 virtue of this property that the stomata are able to regulate the 

 processes of gaseous interchange, in accordance with the varying require- 

 ments of the plant. The power of control is vested in the guard-cells, 

 which, indeed, represent nothing more nor less than a mechanism for 

 regulating the size of the stomatic orifice. The construction and mode 

 of action of this mechanism is not the same in every case. There are, 

 in fact, several distinct types of stomatic apparatus. All of them, 

 however, resemble one another in one point : the opening and closing 

 action can always be referred to the particular structure of the guard- 

 cells. In almost every case the guard-cells take an active part in the 

 process of opening and closing. It is therefore not correct to compare 

 these cells to a pair of curved springs, which become passively com- 

 pressed as the stoma closes under the influence of an external force, but 

 move apart elastically, and thus reopen the stoma, when the pressure is 

 relieved. 



Let us now examine the structure of a typical stoma, such as may 

 be found in any ordinary Dicotyledon and Monocotyledon, with a thin- 

 walled epidermis. The actual aperture of the stoma is enclosed by 

 by two sausage-shaped guard-cells, placed side by side and separated 

 from one another at either end by a thin partition. The average 

 thickness of the inner or ventral face (Bauch-seite) of each guard-cell, 

 i.e. of the side which is nearest to the pore, is considerably greater than 

 that of the outer or dorsal face {Rilcken-scite). As a rule, the ventral 

 wall is provided with two heavily cutinised thickening ridges corre- 

 sponding to its upper and lower edges ; in a transverse section these 

 ridges present the appearance of more or less sharply pointed horns or 

 beaks (Fig. 167). Those ridges which project above and below, overarch 

 two chambers, which may be respectively termed the front cavity 

 ( Vorhof), and the back cavity (Hinterhof) ; the two cavities communi- 

 cate with one another through the pore-passage (Zentrcilspalte , Spalten- 



