INFLUENCE OF SPECIFIC PECULIARITIES ON TRANSPIRATION 237 



and rapidly become flaccid when water is not supplied to them. Never- 

 theless the properties of the cuticle of the leaves and all sub-aerial parts 

 are such as to oppose a marked hindrance to transpiration, whereas in 

 submerged plants, roots, &c., where the conditions of life demand the 

 existence of a readily permeable epidermis, no such check to transpiration 

 is interposed. Consequently when they are exposed to the air such parts 

 rapidly dry" and shrivel. Even in the leaves of our native terrestrial plants 

 a very large proportion, often indeed the greater amount, of the water is 

 lost by diastomatic (intracellular) transpiration. Hence so long as the 

 stomata are open, the leaf surface on which they are most abundant 

 transpires most actively. In the leaves of the oak and the beech, and 

 still more in those of the ivy, the cuticular transpiration is very much less 

 in amount than the diastomatic, and hence the closure of the stomata causes 

 in all these plants a diminution of transpiration which is frequently extremely 

 pronounced. When transpiration is excessive in relation to absorption, the 

 stomata close for the most part before any actual flaccidity has been 

 established. It is only in plants which under normal conditions never 

 suffer from the want of water, that the stomata do not close when sub- 

 jected to artificial drought. 



We may turn our attention first to the leaves, since owing to their 

 large surface-area and the numerous stomata which they contain, they 

 evolve the largest quantities of water-vapour. The young twigs and 

 branches do not usually give off more than i to -^ of the total amount of 

 water-vapour transpired l , and as layers of cork and bark are formed, their 

 powers of transpiration are still further reduced. Since comparatively little 

 water-vapour escapes through the lenticels, the lack of any regulatory 

 power of opening and closing them is of trifling importance 2 (Sect. 31). 



In xerophilous plants the reduction in the relative amount of transpiring 

 surface is produced in various ways. The leaves may become small or may 

 be absent, or they may assume a thick and fleshy character (Sempervivum, 

 &c.), while in the absence of leaves, the flattened stems or branches which 

 take their place (Ruscus, &c.) may be of marked thickness (Cactaceae) 3 . 

 A piece of the stem of Echinocactus would transpire 300 times as much 

 water if it were in the form of thin leaf lamellae 4 . It is the relative 

 amount of water lost which is of importance, and hence it is desirable to 

 measure transpiration in terms of the mass or weight of the entire plant. 



1 Hales, Statics, 1748, p. 580 ; Guettard, Histoire d. 1'Acad. royale, 1748, p. 580; Hartig, Bot. 

 Zeitung, 1863, p. 260; Wiesner u. Pacher, Oesterr. Bot. Zeitschr., 1875, Nr. 5; Eder, Sitzungsb. d. 

 Wien. Akad., 1875, Bd. LXXII, Abth. i, p. 267 ; Burgerstein, 1889, n, p. 14 ; Kny, Ber. d. Bot. Ges , 



1895, P- 374- 



2 Haberlandt, Beitrage z. Kenntniss d. Lenticellen, 1875, p. 17 (Sep.-abdr. a. d. Sitzungsb. d. 

 Wien. Akad., Bd. LXXII, Abth. i. 



3 Cf. Kerner, Pflanzenleben, 1887, Bd. I, p. 302. 



4 Noll, Flora, 1893, p. 355. 



