202 THE MECHANISM OF GASEOUS EXCHANGE 



When transpiration is active, drops of water alternate with bubbles of rarified 

 air in the vessels, and hence when one end of a tracheal vessel is opened the air it 

 contains contracts until it is at the atmospheric pressure, and the contents are pressed 

 towards the closed end of the vessel (or tracheidal cell). If a twig or leaf-stalk is 

 cut under mercury, the latter is sucked into the opened vessels, and partially fills 

 them. This method was first employed by v. Hohnel 1 , who observed the entry of 

 mercury into many of the vessels when transpiring woody and herbaceous plants 

 were used, and frequently found the vessels injected with mercury for a distance of 

 50 to 60 cm. from the cut surface. Similar results may be obtained by employing 

 oil, melted cocoa-butter, or paraffin coloured with alkanna or lamp-black. 



The fact that mercury is sucked in indicates the existence of a marked internal 

 negative pressure, for to overcome the capillary depression of mercury in, for example, 

 the rather narrow vessels of Aesculus hippocastanum of 25 to 30 /n bore, a pressure 

 of 30 to 40 centimetres is needed. As soon, however, as this resistance is over- 

 come and the mercury enters, any further rise in the vessels will depend, just as in 

 a glass tube, upon the volume occupied by the rarefied air in their interior, and there- 

 fore also upon the length of the vessels. Hence the height to which the mercury 

 rises, even when the capillary depression is allowed for, does not form an accurate 

 measure of the rarefication of the internal air, as v. Hohnel erroneously supposed. 

 For the lengths of the tracheal segments differ, and in different plants the tracheae 

 vary from a few centimetres to as much as 3 metres in length 2 . A correct estimation 

 of the negative pressure may, however, be obtained by observing the decrease in 

 volume which the contracting air bubbles undergo when the vessels are opened, 

 and from Schwendener's researches it appears that the pressure of the enclosed air in 

 the tracheae may be as low as one-third to one-fourth of the atmospheric pressure 

 (25 to 19 centimetres of mercury), while similar values have been obtained by Pap- 

 penheim 4 for the tracheides of Coniferae. A complete vacuum is never formed, 

 as Scheit supposed, and this could hardly be expected, since gases diosmose 

 through the walls of the vessels (Sect. 30). Correspondingly rarefied air to that 

 in the dead vessels apparently exists in the other non-living elements of the xylem, 

 although owing to their shortness and narrowness mercury is unable to penetrate 

 them 5 . The existence of a negative pressure may, however, be shown by means 

 of coloured water, which penetrates further when the vessel is opened directly 

 under the coloured solution than when it is at first exposed to air. Haberlandt 



1 V. Hohnel, Uber d. negativen Druck in d. Gefassluft ; Strasburger, Dissertation, 1876, and 

 Jahrb. f. wiss. Bot., 1879, Bd. XI[ > P- 77- Also Strasburger, Ban u. Verrichtung d. Leitungsbahnen, 

 1891, p. 712 ; Schwendener, Sitztingsb. d. Berl. Akad., 1893, Bd. XL, p. 842. V. Hohnel (F. Haber- 

 landt's wissenschaft.-prakt. Unters. a. d. Geb. d. Fflanzenbaues, 1877, Bd. II, p. 122) and others have 

 also employed coloured solutions. In this case, owing to the capillary attraction exerted, the vessels 

 are injected for a greater distance than when mercury is used. 



2 Adler, Unters. iiber d. Langenausdehnung d. Gefassraume, Jenaer Dissert., 1892, and Bot. 

 Centralbl., 1892, Bd. LII, p. 128 ; Strasburger, Uber das Saftsteigen, 1893, p. 50. 



'' Schwendener, Sitzungsb. d. Berl. Akad., 1893, Bd. XL, p. 844. 

 * Pappenheim, Bot. Centralbl., 1892, Bd. XLIX, p. 161. 



5 Scheit, Jenaische Zeitschr. f. Nat.-wi?s., 1885, N. F., Bd. xii, p. 678; Kamerling, Flora, 

 1897, Erganzungsband, p. u. On the existence of living vessels, see Lange, Flora, 1891, p. 52. 



