MECHANICS OF THE STOMA TA. 



249 



The thin lines give the contours of the same cells when the aperture is closed. 

 The figure shows at once that it is not simply a matter of the projection of the inner 

 thin lamella, indicated by d; but that the whole guard-cell changes its form in 

 the process of opening and closing. On account of their firm connection with the 

 neighbouring epidermis cells, this has for its consequence at the same time a dis- 

 placement of both the guard-cells in space also. The other figure (203 d) shows the 

 form of the two guard-cells as seen from the surface ; the guard-cell A is drawn in 

 the condition in which it is when the aperture is closed, while the cell £ repre- 

 sents the form when the aperture is open. Here, however, it is to be noted that the 

 dark part a a, by no means corresponds to the parts marked d in the above figure, 

 but to the ledge there marked a. It is to be noticed in this figure that with the 

 opening of the aperture, the curvature of the outline of the guard-cell B increases ; 

 and that this behaves like a bent tube, which on swelling presses with its two ends 

 against the corresponding parts of the other guard-cell, while the back portion 

 recedes, and the aperture opens. Stomata are found in various plants, it is 

 true, in which the form and position of the guard-cells deviate from those here 



Fig. 203.— Transverse section of a stoma at right angles to the surface of the leaf The thick i 

 of the guard-cells in the open condition, the thin ones the form when close 



show the form 



described : in principle, however, the mechanical relations to be considered are 

 always the same. In the first place, the guard-cells are always thick and 

 strongly cuticularised at their outer and inner walls (Fig. 203 a and d), and the cuticle 

 generally forms more or less strongly projecting cushions or ridges at the places 

 indicated, which surround the outer and inner entrances to the aperture. On the 

 other hand, two thin walls exist in every guard-cell : one, usually the larger, where 

 the guard-cell joins the next neighbouring epidermis cell (^ in Fig. 203); while a thin 

 lamella, generally lower down, bounds the proper aperture of the stoma at d. In 

 the relaxed condition, with slight turgescence, the thick places a and 3 tend to 

 become straight, and parallel with the slit-like aperture; hence they compress 

 the relaxed cell in such a manner that the thin part at d becomes pushed 

 outwards. If, however, the guard-cell absorbs more water, which is facilitated by 

 the thin wall at e, the thin portion d tends to expand in the vertical direction: 

 now, however, it necessarily becomes itself drawn back, since it assumes a more 

 vertical position, as shown by the thick contours in Fig. 203. It is easily seen that 



