THE CONDUCTING CHANNELS 213 



water, so that the diminution of turgidity forms a direct indication of the force with 

 which the cell endeavours to absorb water 1 . The cells bordering upon the water- 

 bearing xylem act by means of the energy thus developed, and in plants well sup- 

 plied with water, the xylem elements retain their fluid contents with but feeble 

 energy, for a slight fall of turgidity in the epidermis suffices to give rise to an out- 

 wardly directed current of water. The attraction of water is obviously dependent not 

 upon the absolute hydrostatic pressure, but upon its relative intensity as compared 

 with that of the regions from which water is withdrawn. Nevertheless, it is self- 

 evident that when a tissue loses water, the cells with feeble osmotic energy will soon 

 collapse, while the more highly osmotic ones will remain turgid for a longer time. 



SECTION 35. The Conducting Channels. 



That water is transferred mainly through the xylem has been confirmed 

 by numerous experiments made since the classical ones by Hales ' 2 . Thus 

 if the continuity of the pith and cortex be interrupted, but the wood left 

 intact, the leaves remain turgid even when transpiration is active. If, 

 however, the wood cylinder is cut through, but the continuity of the cortex 

 and pith retained, withering begins almost as soon as it does in the leaves 

 of a separated branch ; the same thing occurs if a cut branch is allowed to 

 obtain water only by means of immersed strips of cortex or bark, even 

 when the latter, as in the lime-tree, contains a large amount of bast fibres 

 or collenchyma. Hence it follows that none of these tissue elements are 

 capable of rapid transference of water 3 . Similarly, the large amount of 

 fundamental parenchyma which herbaceous plants contain cannot convey 

 water with sufficient rapidity to prevent withering, whereas a few thin fibro- 

 vascular bundles may supply all the water required. 



Since the conductivity of the wood is lost in the inner duramen layers, 

 the water passes in trees only through a greater or less number of the outer 

 annual rings of wood, and not through the central core. In oaks, cherry-trees, 

 pines, &c., a moderately deep ringing incision in the wood suffices, by inter- 

 rupting the continuity of the splint-wood, to cause the withering and death 

 of the parts above, whereas the leafy parts remain fresh and turgid after a 

 similar incision in the beech, birch, and other splint-wood trees, in which the 

 older annual rings retain their alburnum character and can convey water in 

 sufficient amount 4 . Even in such splint-wood the power of conducting water 

 decreases to a certain extent with increasing age 5 , but in palms and in 



1 Sect. 27, and Pfeffer, Studien z. Energctik, 1892, p. 258. 



2 Hales, Statics, 1748, pp. 76, Si, &c. ; Duhamel, Naturgesch. d. Baume, 1765, Bd. n, p. 234; 

 Knight, Phil. Trans., 1801, 11, p. 234, &c. Further literature by Strasburger, Leitungsbahnen, 1891^.515. 



3 Cf. J. Colin, Jahrb. f. wiss. Bot., 1892, Bd. XXIV, p. 172. 



* Knight, Phil. Trans., 1801, n, p. 349 ; Th. Hartig, Bot. Zeitung, 1865, p. 268 ; R. Hartig, Ber. 

 d. Bot. Ges., 1888, p. 222 ; Strasburger, Leitungsbahnen, 1891, p. 515 and the literature here quoted. 



5 R. Hartig, 1. c. ; Wider, Jahrb. f. wiss. Bot., 1888, Bd. xix, p. 82, and Ber. d. Bot. Ges., 1888. 

 p. 406; Strasburger, I.e., p. 592. 



