260 THE MOrEMENTS OF WATER 



it soon sinks again, showing that a backward filtration has taken place 

 against this internal resistance. 



The absence of any exudation of water from the intact surfaces of 

 stems and leaves is clue to the fact that their cuticular and corky invest- 

 ments, as well as their structure, make them permeable with much greater 

 difficulty than the roots, and the permeability of the latter prevents the 

 pressure being raised to an extent sufficient to drive water through the sub- 

 aerial parts, if these possess no water-glands. Without doubt, however, the 

 different parts of the root are not equally active or equally permeable, and 

 hence a local circulation of water may take place in an intact plant, passing 

 either from one branch of the root to another, or even transferring water from 

 one part of the stem to another independently of the transpiration-current 1 . 

 No conclusive experiments have as yet been made to determine whether 

 movements of water produced in this way do actually occur in plants which 

 are not transpiring, and if so, to what extent. 



A similar circulation of water may possibly be exhibited by water-plants, but 

 the experiments which Unger supposed to afford satisfactory proof have been shown 

 by Wieler to be inconclusive, while Sauvageau and Strasburger have not supported 

 their conclusions by experiment 2 . 



As a general rule a lower exudation-pressure is exhibited in mano- 

 meters placed near the summit of a tree, but the pressure does not 

 decrease at all in proportion to the height. Frequently, indeed, mano- 

 meters inserted at the same level, or close to another, register widely 

 different pressures. This was shown first by Hales, and has since been 

 corroborated by Briicke and others 3 . The explanation is that in the 

 different parts of the root and stem, water may be forced into the vessels 

 by the neighbouring living cells, and that a certain amount of time is 

 required to adjust differences of pressure thus produced, so that the 

 water-pressure in a plant is not necessarily distributed as it would be 

 in a glass tube. Hence, it is not surprising to find that when one of the 

 lower branches of a vine is cut away the manometric pressure registered by 

 an upper one sinks to a relatively small extent, and comparatively slowly. 

 Similarly, when an erect vine stem is placed in a horizontal position, a mano- 

 meter attached to it is affected very much less than one would be if it were 



1 Cf. Pfeffer, Studien z. Energetik, 1892, p. 264. 



2 Unger, Sitzungsb. d. Wien. Akad., 1861, Bd. XLIV, p. 367 ; Wieler, 1. c., p. 12 ; Hochreutiner, 

 Rev. gen. de Bot., 1896, T. vm, p. 165; Sauvageau, Compt. rend., 1890, T. cxi, p. 313; Stras- 

 burger, Leitungsbahnen, 1891, p. 930. 



3 Hales, Statics, 1748, p. 67; Briicke, Ann. d. Physik u. Chemie, 1844, Bel. LXIII, p. 195; 

 Hofmcister, Flora, 1858, p. 3, and 1862, p. 117 ; Horvath, 1. c., p. 44 ; N. J. C. Miiller, Bot. Unters., 

 1877, Bd. I, pp. 47, 269 ; Th. Hartig, Bot. Zcitung, 1863, p. 281, and in Luft-, Boden- u. Pflnnzen- 

 kunde, 1877, p. jfiS ; Schvvendencr, Sitzungsb. d. Berl. Akad., iSSrt, p. 583. 



