INFLUENCE OF SPECIFIC PECULIARITIES ON TRANSPIRATION 243 



The cuticle and stomata. The unequal transpiratory activities of the different 

 surfaces of a leaf may be admirably demonstrated by means of pieces of filter-paper 

 saturated with a i to 5 per cent, solution of cobalt chloride and then completely 

 dried. If a piece is laid upon each side of the leaf, and the whole is brought between 

 two glass plates, as in Fig. 31, the paper on the stomatic surface may redden in from 

 a few seconds to a minute or two, while on the upper surface even of thin leaves, if free 

 from stomata (Tropaeolum, Sparmannia, Populus, Pyrus^, a reddening appears only 

 after 3 to 60 minutes. When the cuticle is strongly developed (Ficus elastica, Begonia, 

 Hedera, &c.\ it may take as long as from a few hours to two days before the 

 trifling amount of water required to redden the paper is transpired through the 

 cuticle *. If the cobalt paper is placed beneath a tiny bell-jar attached hermetically 

 by means of fat to a cuticular leaf-surface free from stomata, no water can reach the 

 paper except by cuticular transpiration. The fact that a positive result is always 

 ultimately produced shows that, as has already been said (Sect. 21), the most strongly 

 developed cuticle is not quite, though almost impermeable. It is hardly surprising, 

 however, that no transpiration can be detected through 

 such a cuticle when less delicate methods are employed 2 . 



By using leaves, and comparing the amounts of 

 water transpired when the stomata are open and when 

 they are closed, it may also be shown by the cobalt 

 method that diastomatic transpiration is always pro- 

 nounced in terrestrial plants, and that when the cuticle is 

 very impermeable almost all the transpired water-vapour 

 escapes through the stomata (Sect. 31). When the 

 stomata are closed, it frequently happens that no perceptible difference is observed 

 between the permeability of the cuticle of the two surfaces. 



The high importance of diastomatic transpiration was first proved by Guettard, 

 Bonnet (1754), Eder (1. c.), and Boussingault, who prevented all transpiration from 

 the surface bearing stomata by smearing it with fat or cement, or by placing the 

 leaves with these surfaces in contact with one another 3 . Garreau's results 4 correspond 

 with those already given. In his numerous experiments the arrangement shown 

 in Fig. 32 was adopted. Each of the bell-jars cemented to the upper and the under 

 surface of the leaf contains a vessel of calcium chloride, and the respective increase 



1 Stahl, Bot. Zeitung, 1894, p. 119 ; ibid., 1897, p. 99. [Rosenberg (1. c. on p. 231) states that 

 many Halophytes transpire more actively from the upper surfaces of the leaves than from the lower, 

 although the former contain fewer stomata than the latter.] 



2 Thus cf. Eder, Unters. liber die Aussch. v. Wasserdampf, 1875, p. 102 (Sep.-abdr. a. d. 

 Sitzungsb. d. Wien. Akad., Bd. LXXII) ; Garreau, Ann. d. sci. nat , 1849, iii. seV., T. xm, p. 336. 

 Further literature by Burgerstein. 



3 Guettard, Hist. d. 1'Acad. roy. .de Paris, 1748, p. 579; 1749, p. 292; Boussingault, Agron., 

 Chim. agric., &c., 1878, T. VI, p. 353. Further literature by Burgerstein, II, p. 18. Cf also Stahl, 

 Bot. Zeitung, 1894, p. 129. 



4 Garreau, Ann. d. sci. nat., 1849, iii. ser., T. xm, p. 336. Cf. also von Hohnel, Wollny's Forsch. 

 auf d. Geb. d. Agriculturphysik, 1878, Bd. i, p. 320. A similar method was employed by Unger 

 (Sitzungsb. d. Wien. Akad., 1861, Bd. XLIV, p. 327), and by Risler (Archiv d. sciences phys. et nat. 

 de Geneve, 1871, T. XLII, p. 236). [By employing two leaves, and fixing one of them with its ventral 

 surface upwards, the error due to the different relative positions of the absorbing and transpiring 

 surfaces in the two halves of the apparatus may be partially compensated for.] 



R 2 



FIG 31. 



