194 THE MECHANISM OF GASEOUS EXCHANGE 



as Mohl observed, light generally causes an opening or widening of the 

 stomata, while darkness induces narrowing or closure l , whereas Leitgeb 

 found that in certain plants the stomata open more widely in darkness. 

 Kohl mentions that the stomata in the leaf of Trianea bogotensis rapidly 

 open more widely when exposed to light, and hence are suitable for 

 purposes of demonstration. 



Changes of temperature are much less potent than changes in 

 illumination, for in most cases the former do not markedly affect the 

 width of the stomatal apertures 2 . In certain plants a change of temperature 

 has been found to produce similar effects to those produced by light 8 . 

 Mechanical disturbances appear to be inoperative, while the closure Muller 

 found to be caused by induction shocks is due, according to Leitgeb, to the 

 guard-cells being injured or killed 4 . 



These and similar reactions consequent upon changes of turgidity 

 are not necessarily of great biological significance. The widening of the 

 stomata which strong illumination causes in a turgid plant may be of 

 importance in allowing an increased amount of carbon dioxide to be avail- 

 able for assimilation. On the other hand, the closure of the stomata caused 

 by wetting or by darkness may be of use, in some cases at any rate, by 

 preventing the capillary stomatal pores from being occluded by rain or dew . 



Owing to the reactive powers of the guard-cells, the width of the stoma 

 is unavoidably subject to certain variations under normal vegetative con- 

 ditions. So long, however, as the plant remains fully turgid, the stomata 

 are opened most widely during the day, but for the most part close partially 

 during the night, as has been shown by different investigators 6 . 



It can hardly be doubted that the opening and closing of the stomata are due 

 to variations of turgidity induced by the light or heat rays within the living cells, 

 although the precise manner in which illumination influences turgor is not yet 

 known. According to the nature of the plant and to the external circumstances, 

 darkness may apparently cause either an increase or a diminution of the turgidity of 

 the guard-cells and epidermal cells. In a few of the cases investigated by Leitgeb 

 (1. c., p. 175) the closure of the stomata at night seems to be caused by the more 

 markedly increased turgidity of the surrounding epidermal cells. Frequently, 

 according to N. J. C. Muller (I.e., p. 80) and Schwendener (I.e., 1889, p. 71), the 



1 Schwendener, Monatsb. d. Berl. Akad., 1881, p. 862; Kohl, Transpiration d. Pflanzen, 1886, 

 p. 36 ; Leitgeb, Mitth. d. Bot. Inst. zu Graz, 1886, pp. 137, 182 ; Schafer, Jahrb. f. wiss. Bot., 1888, 

 Bd. xix, p. 196; Stahl, Bot. Zeitung, 1894, p. 124; Schellenberg, ibid., 1896, p. 175 ; Stahl, ibid., 

 *897 ', p. 73- [F. Darwin, Observations on Stomata, Phil. Trans., June 16, 1898.] 



8 Schwendener, I.e., p. 863 ; Kohl, 1. c., p. 38 ; Leitgeb, 1. c., pp. 134, 137. 



* Czech, Bot. Zeitung, 1869, p. 805 ; N. J. C. Muller, Jahrb. f. wiss. Bot., 1872, Bd. VIII, p. 90 ; 

 Eberdt, Transpiration d. Pflanzen, 1889, p. 52. 



* Leitgeb, 1. c., pp. 144, 145 ; N. J. C. Muller, 1. c., p. 96. 



8 Unger, Sitzungsb. d. Wien. Akad., 1861, Bd. XLIV, p. 335 ; Czech, Bot. Zeitnng, 1869, p. 804; 

 Leitgeb, 1. c., p. 149. 



