244 



THE MOVEMENTS OF WATER 



in weight of the two vessels indicates the amount of water exhaled from the upper 

 and under surfaces of the leaf. The tubes m and m' are closed with oil, and serve 

 at the same time as manometers. Of the numerous experiments made by these 

 means, the following examples may be given : 



Transpiration becomes much more active if the cuticle is removed, as experi- 

 ments with apples and cactus stems have shown 1 , but the exposed surface soon 



dries, and other changes occur which cause the 

 rate of transpiration to gradually decrease again. 

 The removal of any waxy covering the leaf 

 may possess also favours transpiration 2 , and at the 

 same time the surface can be more readily wetted 

 with water. Water-vapour is able to pass through 

 a thin layer of fat, though only very slowly 9 ; 

 indeed Boussingault showed long ago that 

 smearing a leaf with fat does not entirely stop 

 transpiration (1. c., p. 357). 



Why it is that syringing the leaves with water 

 markedly increases the transpiratory activity in 

 some cases but not in others, must be left un- 

 answered. It is, however, easy to understand 

 why a decrease in the rate of transpiration on 

 one side of the leaf may frequently, though not 

 always, cause the transpiratory activity to increase 

 on the other 4 . 



It is not desirable to give an account here 

 of the transpiration of different plants or plant 

 organs, and Burgerstein gives a summary of all 

 that is known on this point. Even in many 

 subterranean organs protective adaptions against transpiration are found, such as 

 the corky covering of rhizomes and tubers, or the cuticular skin of bulbs. 



FIG. 32. 



1 Boussingault, Agron., Chim. agric., &c., 1878, T. VI, p. 349; Nageli, Sitzungsb. d. Miinch. 

 Akad., 1861, I, p. 238; Just, Cohn's Beitrage, 1875, Bd. I, p. 11 ; Aubert, Ann. d. sci. nat., 1892, 

 vn, T. xvi, p. 76. Cf. de Candolle, Physiol., T. I, p. 90. 



2 Garreau, Ann. d. sci. nat., 1849, > s>^r.,T. xin, p. 322 ; Haberlandt, Wiss. prakt. Unters. a. d. 

 Geb. d. Pflanzenbaues, 1877, Bd. II, p. 156, and the literature given by Burgerstein, 1. c., II, p. 17. 



3 Laspeyres, Ann. d. Physik u. Chemie, 1878, N. F., Bd. II, p. 478. 



4 Unger, Sitzungsb. d. Wien. Akad., 1861, Bd. XLIV, p. 340; Kohl, Transpiration, 1886, p. 15. 



