THE MECHANISM OF WATER-TRANSPORT 225 



the trachcal elements do actually offer great resistance to mechanical 

 rupture. 



On the other hand, the gaseous tensions existing in the tracheae can 

 hardly be of primary importance in producing the water-current, if this is 

 still shown when the influence of any negative pressure is entirely nullified, 

 as in vacuo. That this is the case is indicated by the fact that neither 

 a diminution nor an increase in the external pressure diminishes or increases 

 the rapidity of the water-current 1 . Hence, although the negative pressure 

 existing in the tracheal channels must be of importance in regulating the 

 flow of water, it cannot be the actual cause of it 2 . It must be remembered, 

 however, that the tracheae become completely filled with water when the 

 supply is sufficiently abundant, even when the plants are in a vacuum, and 

 the presence of air-bubbles, which must be dissolved away, acts merely as 

 a temporary hindrance to complete saturation. In herbaceous plants the 

 tracheae and tracheides may occasionally be completely filled with water 

 in the ordinary course of events 3 , and it would be well to learn whether 

 a similar filling of the tracheae may take place as far as the summit of 

 a lofty tree, or whether continuous water-columns of this length cannot 

 exist, and if so, whether the formation of a chain of columns of water and 

 air is a necessity in tall plants. 



It is possible that in this short sketch important experiments and facts 

 have been overlooked, while unimportant details have been made un- 

 necessarily prominent. This is, however, unavoidable as matters are at 

 present, and whatever the final solution of the problem may be, the causes 

 at work must necessarily be such that the conducting channels become filled 

 to a certain extent with water, and that by a disturbance of equilibrium 

 a corresponding flow of water is induced. It is, however, impossible to tell 

 from these simple essential conditions whether the transference of water 

 takes place with or without the active aid of living cells 4 . 



Historical. Many facts concerning the transpiratory current were established 

 by Grew, Malpighi, Mariotte, Woodward, and others, but it was the masterly 

 researches of Stephen Hales 5 which laid the foundation of our present knowledge. 



1 Strasburger, Ban n. Verricht. d. Leitungsbahnen, 1891, p. 793 ; Uber das Saftsteigen, 1893, 

 p. 64; Dixon and Joly, Phil. Trans., 189;, Vol. CLXXXVI, p. 564. 



2 Variations in the amount of air present are shown almost solely in the alburnum wood : 

 Hartig, Ber. d. Bot. Ges., 1888, p. 223, and the literature given in Sect. 37. On the relative amounts 

 of air and water present in wood, see also Sachs, Vorles. Uber Pflanzenphysiol., 1887, 2. Aufl., p. 219. 



3 Cf. p. 220 and Sect. 32 ; also Schwendener, Sitzungsb. d. Berl. Akad., 1892, Bel. XLIV, p. 931 ; 

 Strasburger, I.e., 1893, p. 25. 



* [H. H. Dixon (Proc. R. Irish Soc., 1898, Vol. IV, pp. 618, 627) concludes that the latter is 

 the case, whereas Schukowsky and Wottschal (Nat. Hist. Soc. of Moscow, Dec. 1897) consider that 

 the ascent of water is a purely physical phenomenon. Wottschal indeed states that the movements 

 of water in a column of sand are similar to those in the stem of a tree. Cf. Bot. Centralbl., 

 Bd. i.xxvn, 1899, p. 337.] 



5 Hales, Statics, 1748: rf. Sachs, History nf Botnny fGarnsey and Balfonr\ 1886, p. 500; 

 PFEFPER O 



