222 THE MOVEMENTS OF WATER 



transferred through the substance of the tracheal walls with sufficient 

 rapidity, over even comparatively short distances, to supply transpiring 

 leaves with the water they require, and hence in stems injected with 

 gelatine, &c., dyes pass upwards with extreme slowness 1 . 



As Kohl 2 has shown, withering ensues when the lumina of the tracheae 

 are partially closed by the pressure of a compression screw, while when 

 this is removed the flow of water becomes normal again. It must be 

 borne in mind that fluid gelatine does not hinder the transit of water, and 

 hence the normal current of water recommences if the solidified injected 

 gelatine is melted by raising the temperature to about 3o-32 C. Dis- 

 regarding the intercellular spaces, which do not concern us here, the 

 gelatine penetrates into and blocks up the cavities of the tracheae, the 

 tracheides, and eventually the bast fibres as well. Hence it follows that 

 all the other tissue-elements together, living or dead, are unable to convey 

 the water which the plant requires ; at the same time the supposition put 

 forward by Sachs 3 that the water travels most rapidly in the lignified walls 

 of the wood-tracheae is decisively negatived, for the pressure and injection 

 experiments show that the excessive conductivity which Sachs' theory pre- 

 supposes is not actually possessed by the walls of the wood vessels. A most 

 marked conductivity would moreover be necessary, for a trifling difference of 

 potential suffices to induce a flow r of water, so that a very slight force must 

 overcome the enormous resistance to filtration interposed by a length of 

 20 metres or more of lignified cellulose. For these and other reasons it 

 is impossible that the water could travel in the substance of the walls with 

 sufficient rapidity to supply the requirements of transpiration, although the 

 force of imbibition (Sect. 1 2) would suffice to raise the water higher than 

 the summit of the tallest tree 4 . 



The tracheal channels are occupied in transpiring plants by a chain 

 of water-columns and air-bubbles (Jamin's chain) 5 , and the water can either 



been obtained by Strasburger (Ban u. Verricht. d. Leitungsbahnen, 1891, p. 541) with gelatine, and 

 by Dixon and Joly Ann. of Bot, 1895, Vol. IX, p. 403) with gelatine and paraffin. 



1 Dixon and Joly (1. c.) have also showed that after injection with paraffin, water does actually 

 travel upwards through the tracheal walls, but only very slowly. 



2 Kohl, Bot. Zeitung, 1885, p. 522, and Transpiration d. Pflanzen, 1886, p. 118; F. Darwin 

 and Phillips, Proc. of the Camb. Phil. Soc., 1886, Vol. V, p. 364; Strasburger, 1. c., p. 603. As 

 Russow has shown (Bot. Centralbl., 1883, Bd. Xin, p. 99; cf. also Godlevvski, Jahrb. f. wiss. Bot., 

 1884, Bd. XV, p. 628% the lumina are not, as a general rule, closed when the stem is sharply bent. 

 Dufour (Arb. d. Bot. Inst. in Wiirzburg, 1884, Bd. in, p. 41) erroneously concluded from such 

 experiments that he had established a proof of the imbibition theory. 



3 Sachs, Arb. d. Bot. Inst. in Wiirzburg, ] 878, Bd. II, pp. 148, 291 ; Vorles. iiber Pflanzenphysiol., 



1887, 2. Aufl., p. 2OI. 



* See Godlewski, Jahrb. f. wiss. Bot., 1884, Bd. xv, p. 602 ; Schwendener, Sitzungsb. d. Berl. 

 Akad., 1886, Bd. XXXIV, p. 591 ; Pfeffer, Studien zur Energetik, 1892, p. 258. 



'' From the literature quoted in Sect. 32, it may be seen that this Jamin's chain persists even 

 when but little water is present. 



