MOVEMENTS AND PRESSURE OF THE INTERNAL GASES 203 

 has shown that a negative pressure exists in the elements of the central cylinder of 



a moss 1 . 



These gaseous tensions arise, as v. Hohnel has shown, in direct connexion 

 with the changing amounts of water present. Hence, when fully saturated with 

 water, no negative pressure exists in vessels or tracheides (as the manometer also 

 shows, Fig. 24, p. 201), whereas it appears as soon as transpiration commences, and 

 is most marked when the latter is fully active. As the water in the chain of air 

 and water columns (Jamin's chain) which fill the vessels increases again, so does the 

 negative pressure of the enclosed air decrease <i , and hence such negative pressure 

 often disappears during the night in herbaceous plants whose vessels may become 

 filled with water. In woody plants as well, as v. Hohnel has shown, the negative 

 pressure gradually disappears when transpiration ceases. 



The rarefication of the enclosed air which results from the removal of water is 

 only very gradually neutralized, as v. Hohnel recognized (1879, p. 76), owing to the 

 slowness with which gases diosmose through lignified walls (Sect. 21, 30). It is still 

 uncertain whether the daily variations in the percentage of water act directly, as v. 

 Hohnel supposed, or whether other factors co-operate in maintaining a negative pres- 

 sure within the vessels. Obviously the pressure must finally become uniform, unless 

 the difference of potential is continually restored, and hence it is easy to understand 

 that in leafless trees during winter the negative pressure in the air of the wood 

 vessels is reduced to a minimum or to nil 3 . Bearing in mind the different factors 

 which influence or induce the movement of water, it is not surprising to find that 

 the pressure may differ in neighbouring vessels, though it is approximately the 

 same at different levels of the same tree 4 . In the root the negative pressure is 

 frequently less marked than in the stem 5 . 



Isolated twigs, after being exposed to air for a time, again show a negative 

 pressure when a new surface is exposed near to the old one. This is partly due to 

 the reopening of tracheides which have gradually become closed, and partly owing 

 to the opening of others hitherto intact. A closure rendering possible the re-establish- 

 ment of a negative pressure in opened vessels may take place by means of an 

 exudation of slimy material, or by the growth of bacteria on the cut surface, or by 

 other means as well 6 . 



The positive pressure of the intercellular air is due to the excretion into the 

 intercellular air-spaces of a portion of the oxygen produced by the decomposition of 



1 Haberlandt, Jahrb. f. wiss. Bot, 1886, Bd. XVII, p. 416. 



3 V. Hohnel, Jahrb. f. wiss. Bot., 1879, Bd. XII, p. 121. On the contents of the vessels, cf. 

 Bohm, Bot. Zeitung, 1879, P- 2 55 ! v - Hohnel, 1879, 1. c., p. 121 ; Volkens, Jahrb. d. bot. Gart. in 

 Berlin, 1883, Bd. n, p. 181 ; Schwendener, Sitzungsb. d. Berl. Akad., 1886, p. 566; Strasburger, 

 Bau u. Verrichtung d. Leitungsbahnen, 1891, pp. 685, 695, and Uber das Saftsteigen, 1893, p. 55. 



3 V. Hohnel, I.e., p. 115; Strasburger, 1891, I.e., p. 715. Contrary to the negative results 

 of v. Hohnel, Bohm (Ber. d. Bot. Ges., 1889, Generalvers., p. 52) found a certain negative pressure 

 to exist in winter, and hence it is possible that exceptions may exist. 



4 Schwendener, Sitzungsb. d. Berl. Akad., 1892, p. 923; 1893. p. 844; Strasburger, Uber d. 

 Saftsteigen, 1893, p. 56. 



5 Strasburger, Bau u. Verrichtung d. Leitungsbahnen, 1891, p. 715. 



8 V. Hohnel, 1. c., 1876, p. 20, and Bot. Zeitung, 1879, p. 320; Strasburger, 1891, I.e., p. 714. 



