192 THE MECHANISM OF GASEOUS EXCHANGE 



whereas in the case of our native orchids, and a few other plants, wetting with water 

 caused the stomata to open more widely. Since the isolated guard-cells always react 

 in the same manner, these differences can only result from the opposing action of 

 the surrounding tissue, and, as Mohl found, this is frequently very'feeble in character. 

 Hence arise the contradictory results obtained by different workers, which, however, 

 differ quantitatively only, and are perhaps partially due to the influence exerted by 

 the cultural conditions. Mohl observed that when flaccid leaves of Amaryllis 

 formosissima with closed stomata are placed in water, the stomata open at first and 

 become fully expanded in about five minutes, then narrowing and ultimately closing 

 again. This is owing to the antagonistic action of the guard-cells and the sur- 

 rounding epidermal cells, and to the unequal rapidity with which these absorb 

 water. Stahl J observed that in the drooping leaves of certain plants the stomata 

 remain open in air saturated with moisture, though when transpiration is permitted 

 they completely close, but it has yet to be determined whether this is due to the 

 antagonistic action of the epidermal cells or to other causes. 



From the above researches it appears that as a general rule it is the neighbour- 

 ing epidermal cells which act antagonistically to the guard-cells, but it is not sur- 

 prising to find that apparently in many cases some influence is exerted by the union 

 of the epidermis with the tissues beneath (see also Mohl, 1. c., pp. 703, 717). 

 According to Benecke 2 , when subsidiary guard-cells are present, these are of import- 

 ance in shielding the guard-cells proper from pressure and tension of external 

 origin. 



The mechanism of the opening movements. The movements of the stomatal 

 apparatus caused by changes of turgidity are due to its form and structural relation- 

 ships. This was originally recognized by Mohl, but the relationships actually 

 existing were first made clear by Schwendener. From his researches and those 

 of his pupils s , it appears that precisely the same mechanism is not employed in all 

 cases to produce the opening movement which follows a rise of turgidity. It must, 

 however, suffice to indicate a few only of these relationships. 



In most cases the opening is probably due to the concave surface of the 

 guard-cell (d, Fig. 20) being less extensible than the convex one (e\ and hence elon- 

 gating less as the turgidity increases. The same result is produced when air is forced 

 into an india-rubber tube having one side thicker than the other. By employing two 

 such pieces of tubing, a model may be constructed in which, when the india-rubber 

 walls of the ' guard-cells ' are subjected to increased tension, they separate from one 

 another and open the 'stoma.' This result is attained in the guard-cells by an 

 increased thickness of the less tensile walls, as can be seen in the transverse section 

 of the stoma of Helleborus shown in Fig. 20. In this particular case it is also 



N. J. C. Miiller, Jahrb. f. wiss. Bot, 1872, Bd. VIII, p. 75 ; Leitgeb, Mitth. d. Bot. Inst. in Graz, 



1886, p. 125, &c. 



1 Stahl, Bot. Zeitung, 1894, p. 121. 



z Benecke, Bot. Zeitung, 1892, p. 538. 



3 Schwendener, Monatsb. d. Berl. Akad., 1881, p. 833; Sitzungsb. d. Berl. Akad., 1889, p. 65 

 (Gramineae and Cyperaceae). A concise rtsumt is given by Haberlandt, Physiol. Anat., 1896, 

 2. Aufl., p. 396. Also Haberlandt, Jahrb. f. wiss. Bot., 1886, Bd. XVII, p. 461 (Mosses), and Flora, 



1887, p. 106; Schafer, Jahrb. f. wiss. Bot, 1888, Bd. XIX. p. 200. 



