INFLUENCE OF SPECIFIC PECULIARITIES ON TRANSPIRATION 239 



differences of osmotic energy correspond to comparatively small differences 

 of vapour tension, and hence exercise but little effect upon transpiration l . 

 The latter may, however, be markedly diminished if the leaves become 

 coated by an investing gelatinous covering which dries and forms a more or 

 less impermeable film on their outer surfaces (Sects. 23, 30). The film 

 covering the so-called ' varnished ' leaves may act in this manner, as also 

 do the waxy films found on the epidermis of many plants. 



During development various adaptative modifications and peculiarities 

 arise by means of which transpiration may be regulated according to the 

 external conditions and the necessities of the plant. Thus, neglecting the 

 effects of the increase in the bulk of the leaf, the development of a cuticle 

 causes a diminution in its transpiratory powers, whereas the concomitant 

 development and opening of the stomata acts in the opposite manner, 

 until with the autumnal closure all diastomatic transpiration ceases. Hence, 

 under similar external conditions, a transpiratory maximum occurs in each 

 plant at a particular stage of development, and the curve of transpiratory 

 activity may frequently exhibit secondary maxima 2 . 



As is well known, the supply of water has a great influence on growth 

 and development, and a moderate deficiency of water causes transpiring 

 plants to develop peculiarities which tend to the more perfect utilization 

 of the supply and also to the limitation of the loss by transpiration. The 

 entire shape, the slow growth, and the small size of those plants which 

 grow in dry habitats, are all produced in response to the special conditions 

 under which they exist. Even the shedding of a number of the leaves, 

 which occasionally occurs during a summer drought, may be of biological 

 importance as a last attempt to avoid irretrievable injury. 



The cuticle is, as a general rule, more strongly developed when there 

 is a scarcity of water, or even when the supply is abundant but transpiration 

 active, than when a damp atmosphere or the absence of sunlight reduces 

 the latter to a minimum. The occurrence of wax impregnating the cuticle 

 is much less conspicuous in leaves grown in moist air or in those which are 

 submerged, even in the cases of plants whose leaves are normally covered 

 by a waxy bloom 3 . Changes of this kind are not, however, always due 



1 The presence of a solution of tannic acid cannot diminish the rate of transpiration more 

 markedly than any other saline solution of equivalent osmotic value. For this supposed action of 

 tannic acid, see the literature given by Burgerstein, 1. c., II, p. 63. 



a For leaves see v. Hohnel, Wollny's Forschungen auf d. Geb. d. Agriculturphysik, 1878, Bd. 

 1, p. 299 ; Aubert, Ann. d. sci. nat., 1892, vii. ser., T. xvi, p. 85 ; Stahl, Bot. Zeitung, 1894, p. 199. 

 Further literature : Guettard, Histoire d. 1'Acad. royale de Paris, 1748, p. 579, and 1749, p. 292 ; 

 Fleischmann, Versuchsst., 1867, Bd. IX, p. 182 ; Vesque, Ann. d. sci. nat., 1877, vi. ser., T. IV, 

 p. 89; N. J. C. Miiller, Bot. Unters., 1877, Bd. I, p. 155 ; Bonnier et Mangin, Ann. d. sci. nat., 

 1884, vi. ser., T. xvn, p. 295 (Fungi). For further literature see Burgerstein, 1. c., II, p. 24. 



3 Kohl, Transpiration, 1886, p. 113; Tittmann, Jahrb. f. wiss. Bot., 1896, Bd. XXX, p. 116. 

 Cf. also Burgerstein, 1. c., II, pp. 45, 61, and Sect. 21. 



