DIV. I 



MORPHOLOGY 



in shape of the guard cells to be effected without hindrance from the surrounding 

 cells (cf. Fig. 47 B). The guard cells, as is seen in Fig. 45 A, are often surrounded 

 by special cells called subsidiary cells ; these may be less thickened or shallower 

 than the other epidermal cells. 



Differences are found in the construction of the guard cells and in the 

 mechanism of opening and closure of the stoma which depends upon this. Two main 

 types of stoma may be distin- 

 guished but they are connected by 

 intermediate forms. In the first 

 the change in form of the guard 

 cells takes place mainly in the 

 tangential direction, parallel to 

 the epidermal surface ; in the 

 second in the radial direction at 

 right angles to the surface^ TYPE 

 I. According to the form of the 

 guard cells the pore is opened in 

 various ways, (a) The type of 

 the Amaryllidaceae (Fig. 47) is 

 found in the majority of Mono- 

 cotyledons and Dicotyledons. 

 The dorsal Avail of each guard cell 

 (Fig. 47 B) is unthickened, while 

 the ventral Avail (towards the 

 pore) is thickened and usually 

 shoAvs the upper and lower 

 thickening bands. When the 

 cell becomes turgid the thin 

 dorsal Avail is more stretched than 

 the thickened Avail, and the cell, 

 which in the flaccid condition 

 Avas almost straight, becomes 

 curved in the tangential plane 

 to a semilunar shape, (b) The FIG. 49. 



type of the Gramineae (Fig. 48) FIGS. 47-49. Types of Stomatal Apparatus. The thick 

 is met Avith in the Gramineae and lines indicate the form of the guard cells in the open 

 Cyperaceae The guard cells are condition, the thin lines when the stoma is closed, 

 dumb-bell-shaped; the widened ^"ZgZZZSZ* + *"*"*" 

 ends being thin-Availed, while the PIG> 48t _ T yp e O f the Gramineae with the two subsidiary 

 narrower middle region has both cells. A, Surface view. B, Transverse section, 

 the outer and inner \valls strongly FIG. 49. J/ntum-type in transverse section. (After 

 thickened (Fig. 48 B}. When HABERLANDT.) Further description in the text, 



the turgor increases the stiff 



middle portion of the guard cells are separated from one another by the expansion 

 of the oval thin-walled ends of the cells. TYPE II. If/mm -type (Fig. 49) is 

 found in some Mosses and Ferns. In this the ventral walls of the guard cells 

 are thin Avhile the dorsal Avails are thickened. When the turgor of the guard cell 

 increases, the outer and inner walls are separated from one another, thus lessening 

 the projection iuAvards of the ventral Avail and opening the pore. The position 

 of the dorsal Avail remains unchanged. 



The stomata are formed by the division of a young epidermal cell into two cells 

 of unequal size, one of which, the smaller and more abundantly supplied Avith 



