280 



THE MOVEMENTS OF WATER 



dye probably passed, not through the epithelial cells, but through the spaces between 

 them. In Aroids the space beneath the water-stoma takes the form of a long intra- 

 cellular canal that runs parallel to the marginal bundle, and thus presents a large 

 surface through which water may filter. According to Haberlandt (1. c., 1895, p. 89) 

 stomata may function as water-pores in young grass leaves, but in old ones it 

 has hitherto been supposed that the water-pores are merely cracks which appear 

 in the epidermis. It is, moreover, obvious that water may be driven through 

 ordinary stomata whenever the intercellular spaces are injected with water (Moll). 



Since the excretion of water from water-pores is largely or entirely dependent 

 upon the internal hydrostatic pressure, it will be influenced by the process of 

 development, and by external agencies, to an extent corresponding to the effect they 

 produce upon the general activity of exudation. The excretion of water is dependent 

 upon the amount present, but no critical experiments have been performed to 

 determine the influence which external agencies may exert '. 



Duchartre and Unger 2 observed that the excretion of water from Aroids 

 decreases when the plant is exposed to light, but this is probably because the rate 



of transpiration increases, and 

 similar objections may be made 

 to the results obtained by 

 Gardiner (1. c., p. 42), as well 

 as to the observations made 

 upon the daily variations in 

 the amount of water present 

 in the pitchers of Nepenthes, 

 which are supposed to be due 

 to the action of light '\ A few 

 cases have, however, already 

 been mentioned (Sect. 47) in 

 which the excretion of water 

 is undoubtedly excited by an external stimulus. 



The permeability of the cells through which the water filters is naturally also 

 of importance in determining whether any excretion of water is possible, as well as 

 the rate at which it may occur. It must, therefore, be determined in each case by 

 empirical investigations whether the water-pores of old leaves cease to excrete water 

 because the activity of the cells which generate the exudation-pressure decreases, or 

 because the epithelial cells offer more resistance to the passage of water through 

 them. Moreover, it is only in certain plants that even a maximal hydrostatic 

 pressure is able to cause drops of water to fall from the leaves. The most active 

 excretion of water as yet observed occurs in certain Aroids ; thus Musset saw as 

 much as eighty-five drops fall from the tip of the leaf of Colocasia esculenta in one 



FIG. 39. Fuchsia hybrida (horf. Longitudinal section through 

 \vater-pori'. (x 100). 



1 See Volkens, I.e., p. 172; de la Rue, But. Zeitung, 1866, p. 317; Duchartre, Ann. d. sci. 

 nat., 1859, iv. scr., T. XII, p. 267; Schmidt, Linnaea, 1831, 13d. vi, p. 65. 



' J Unger, Silzungsb. d. Wien. Akacl., 1858, Bd. xxvm, p. 15. 



3 Korthals, &c., quoted by Wuuschmann, Ober die Gattung Nepenthes, 1872, p. 28; Goebel, 

 Pflanzenbiol. Schilderungen, 1891, Th. ii, p. 160; 1893, Th. iii, p. 168 ^Sarracenia). 



