ELEMENTARY STRUCTURE. 43 



lips and mouth of an animal, and hence the name Stomata 

 applied to such orifices (a-rSfia, a mouth). The bordering cells 

 of the orifice have been called "stomatal cells," or "pore cells," 

 or " guard-cells," and have the power of opening or closing 

 the orifice Avhich they guard according to circumstances, as will 

 be explained when treating of the functions of stomata. 

 Instead of two stomatal cells, we sometimes, although but 

 rarely, find four, or even more ; thus, in some of the Liverworts 

 (HepaticacecB), the stomata are rounded apertures between the 

 epidermal cells, surrounded by three or more tiers of stomatal 

 cells, each tier being itself composed of four or five cells, the 

 whole forming a kind of funnel or chimney (fig. 89). 



Upon making a vertical section through the stomata, we find 

 generally that the stomatal cells are placed nearly or quite on 

 a level with those of the epidermis. In other cases, however, 

 and especially when situated upon leaves of a leathery or hard- 

 ened texture, the stomatal cells are below the epidermal ones, 

 while in some rare instances again they are above them (fig. 89). 

 The stomata vary in form and position in different plants and 

 in diff'erent parts of the same plant, but they are always the 



Fig. 90. Fig. 92. Fig. 93. 



Fig. 91. 



Fig. 90. Vertical section of the epidermis of Leucaclendron decorum, showing 

 e, e, the epidermal ceils, with the stomatal cells, s, with elevated mar- 

 gins, m,m Fifj. 91. Vertical section of the epidermis of the Iris. s. 



The stomate. e. Epidermis, p. Parenchyma beneath the epidermis. 7. 



Intercellular space into which the stomate opens- Fig. 92. Epidermis 



of Buniex Acetosa, with rounded stomata, a Fig. 93. Square stomate 



of Yucca gloriosa. 



same in any particular part of a plant. The most common 

 form is the oval (figs. 76, 77, and 88); in other instances they 

 are round (fig. 92, a) or square (fig. 93). They are either 

 placed singly upon the epidermis, at regular (fig. 76), or irregu- 

 lar intervals (fig. 92), or in clusters, the intervening epidermis 

 having none (fig. 94). In the Oleander (Nerium Oleander') we 

 find httle pits beneath the epidermis of the under surface of the 

 leaves which contain a number of hairs, and A^ery small stomata 

 on their sides (fig. 95). 



