238 THE MOVEMENTS OF WATER 



The rate or intensity of transpiration is found by estimating the amount of 

 water given off from a unit of transpiring surface in a given time. The 

 transpiratory intensity is sometimes greater in a turgid succulent leaf than 

 it is in an ordinary thin one l , but in comparison with their respective bulk 

 or weight, the transpiration is very much greater in the latter case than in 

 the former. Moreover the great reduction of the cuticular transpiration 

 enables xerophilous plants to reduce the loss of water to a minimum when 

 necessary. The intensity of the cuticular transpiration of young roots, &c. 

 is on the other hand so pronounced that they are undoubtedly frequently 

 able to transpire more actively than thin leaves with numerous stomata. 



Any increase in the surface-area due to the formation of outgrowths 

 tends to favour the rate of evaporation, but nevertheless the presence of 

 strongly cuticularized scales or overlapping hairs may exercise a certain 

 check upon transpiration -. A covering of dead hairs always lessens 

 transpiration, but this means is only employed to a limited extent by 

 plants. In certain cases leaves are able, by rolling themselves up, to 

 diminish the amount of surface exposed, and thus to check the rate of 

 transpiration when necessary 3 . The denseness of the foliage, the phyllo- 

 taxis of the leaves, and their position with regard to the incident rays of 

 light are also of importance. The sun's action may be tempered by the 

 assumption of a vertical position, and the active paraheliotropic movements 

 which certain leaves or leaflets can execute may be of importance from 

 this point of view 4 . As the amount of water present decreases, transpira- 

 tion becomes less and less active, and ceases when the air-dried parts 

 contain only hygroscopic water. 



So long as the plant remains turgid, the physical action of an increasing 

 concentration of the cell-sap does not diminish the rate of transpiration to 

 any marked extent 5 . Any change in the osmotic energy causes a corre- 

 sponding change in the amount of water imbibed by the cell-wall, and hence 

 affects the rate of evaporation from the latter (Sect. 27), but the possible 



1 For examples see Aubert, Ann. d. sci. nat., 1892, vii. se"r., T. XVI, p. 80; also Burgerstein, 

 1. c., n, p. 24. 



* For literature see Burgerstein, 1. c.,n, p. 457; Kerner, Pflanzenleben, 1887, Bd. I, p. 289. 

 On chalky and saline incrustations see Sect. 23. 



3 See Burgerstein, n, p. 60 ; Kerner, 1. c., p. 134 ; Kihlmann, Pflanzenbiol. Stndien, 1890, p. 105. 

 The heating action of sunlight will hardly be decreased to any appreciable extent by the admixture 

 of less diathermanous ethereal oils with the vapour evolved. Literature : Burgerstein, II, p. 457. 



* Burgerstein, 1. c., n, p. 260. [This is by no means always the case. Thus in Albizta 

 safonana, Calliandra haematocephala, Dalbergia linga, and Cassia man tana, from the distribution 

 of the stomata and the character of the paraheliotropic position which the leaflets assume, it 

 follows that the rate of transpiration can hardly be checked, and may even be increased if the 

 stomata remain open. It is only in such plants as Mimosa, Robinia, Acacia, &c., that transpiration 

 is markedly checked when the leaflets fold together with their stomatic surfaces apposed in assuming 

 the paraheliotropic position (Ewart, The Effects of Tropical Insolation, Annals of Botany. 1897, 

 PP- 455-9)-] 



s See Aubert, Ann. d. sci. nat., 1892, vii. ser., T. XVI, p. 64. 



