TISSUES AND SIMPLE ORGANS. 



137 



Second type : Helleborus. Front cavity immovable, back cav- 

 ity undergoing great changes in size and position. The changes in 

 form of the lumen of the guard-cell as seen in cross-section are 

 such that in the non-turgescent state the outline of cell-wall pre- 

 sents the form of a scalene triangle ; in the turgescent state it as- 

 sumes more nearly the form of an isosceles triangle (see Fig. 76). 

 Chief mechanical change : hinge- joint movement of the thin areas 

 of the guard-cells near the central passage accompanied by a similar 

 movement of the distal wall near the cuticular joint. The entire 

 guard-cell may also become curved. There will be no difficulty in 

 finding forms intermediate between the first and second types. 



Third or Gramineous type. The frequently much- elongated 

 middle portion of the guard-cell is thick- walled and passive. The 

 lumen of the guard- cell in cross- section 

 through the middle presents the ap- 

 pearance of a wedge placed transversely 

 (Fig. 77). The movements due to tur- 

 gor are manifest in the expanded thin- 



walled lower ends (subsidiary cells) of 

 . _ ., ,__,. 



the guard-cells (Figs. 78 and 79). The 



open central passage (stoma) is bounded 



by parallel straight lines, formed by the outline of the above-men- 



tioned middle portions of the guard-cells (Fig. 79). The subsid- 



G - W- Vertical section 

 through the storaa of Garex 

 leporina. 



FIG. 78. Radial longitudinal section through a guard-cell of Triticum. 

 (After Schwendener.) 



iary cells (n) perform the function of 1 , a " membranous hinge ' ' 

 similar to the thin cell- wall areas of the other types ; 2, in a few 

 instances, verified by experiment, they assist in closing the pores 

 during the turgescent state, since the central pore has been observed 

 to remain open even after the subsidiary cells and guard-cells were 

 killed. 



The exceptions referred to under type II. are not contradictory 

 of the following general statement : Increasing turgescence of the 

 guard- cells is the force which causes the opening of the pores, and, 



