Transpiration ami the Ascent of Sap. \^ 



it is found that the leaves above the killed portion sooner or later 

 fade and wither. The vitalists' interpretation of this observation is 

 that, when the vital actions of the wood parenchyma and raedullary- 

 raj^-cells are removed, the supply of w^ater to the leaves above is so 

 reduced, that they fade and dry from want of water. It is significant 

 that the discoverers of the phenomenon saw in it no support of the 

 vital hypothesis. Weber ^) recog-nises that the reduction of the 

 water supply may be attributed to the stoppage of the tracheal tubes. 

 Janse^j although supporting Godlewski's vital theory, quickly 

 saw that this observation, being traceable to the blocking of the 

 supply tubes, could not be quoted in support of the vital hypothesis. 

 Finally V e s q u e ^) sees no support in it for the vital theory. 

 Ursprung'*), however, the most recent champion of the vital hypo- 

 thesis relies on the observation with great confidence to support his 

 view, but even his observations show that, out of some twenty species 

 in which the phenomenon was observed, in fourteen stoppages in the 

 conducting tubes were actually observed after heating — usually 

 above the heated region. Ursprung, in spite of this, maintains that 

 the reduction in the water supply is not caused by stoppage because 

 (1) the leaves may begin to die before the stoppage of the tubes 

 is observed; (2) even when stoppages do occur fading may be post- 

 poned; (3) fading does not occur when a piece of the wood is removed 

 from an uninjured branch corresponding in size to the plugged portion 

 of the heated branch. 



It is to be noted that the failure to observe microscopically these 

 stoppages does not negative their existence. A transparent material 

 might form quite an effective plug in the tubes or render the 

 transmitting pits almost impermeable, while it might be almost or 

 quite invisible. Such a transparent substance W e b e r ^) did actually 

 detect in his experiments on Picea excelsa. This investigator's obser- 

 vation, that the resistance of the region immediately above the killed 



^) C. A. Weber, Ueber den Einfluss höherer Temperaturen auf die Fähigkeit 

 des Holzes den Transpirationsstrom zu leiten. Ber. d. Deutsch. Bot. Gesell., 3, 

 pp. 34Ô— 371. 1885. 



^) J. M. Janse, Die Mitwirkung der Markstrahlen bei der Wasserbewegung 

 im Holz. Jahrb. f. Wiss. Bot. 1887. 



') J. Vesque, Sur le prétendu rôle des tissus vivants du bois dans l'ascension 

 de la sève. Comptes rendus, 101, 2, 1885, p. 757. 



*) A. Ursprung, Die Beteiligung lebender Zellen am Saftsteigen. Jahrb. f. 

 wiss. Botanik, XLII, 1906, p. 522, cf. Idem, Ueber die Wasserversorgung der Pflanzen. 

 Biol. Centralbl., XXVII, 1907, Idem, Untersuchungen über die Beteiligung lebender 

 Zellen am Saftsteigen. Beih. z. Bot. Centralbl., 1904, Idem, Ueber die Ursache des 

 Welkens. Beih. z. Bot. Centralbl , 1907, und Idem, Abtötungs- und Ringelungs- 

 versuche an einigen Holzpflanzen. Jahrb. f. wiss. Bot., XLIV. 1907. 



*) C. A. Weber, loc. cit. 

 Progressas rei botanieae III. 2 



