Trauspiration and the Ascent of Sap. 27 



objection to this hypothesis resides in the fact that such a motion, 

 being of the nature of diffusion, must needs be very slow, and, being 

 confined to the thickness of the walls of the conducting tissues, would 

 be able to transmit water in vanishingly small quantities. Imbi- 

 bitionists met this argument, rather lamely, by asserting that water 

 in the imbibed state is surpisingly mobile! 



This fanciful statement was soon disproved by experiments 

 showing that the imbibed walls alone are inadequate to transmit the 

 transpiration stream. Thus, by filling the opened lumina of a cut 

 branch with clogging materials, but leaving the cut surface of the 

 walls to absorb and transmit water, it was found that the branch 

 faded long before a control which had its lumina free as well as its 

 walls to transmit water. For this experiment Elfving^) and Vesque^) 

 used cacaobutter, Scheit^) gelatine coloured with eosin, Errera"*) 

 and Strasburger^) gelatine and Indian ink, and finally Joly and 

 the author ^) obtained the same result after blocking the lumina with 

 paraffin, gelatin and COg gas. The results of the two last, however, 

 showed that a slight upward movement did actuallj^ take place in 

 the walls but so insignificant that it could not be of any importance 

 in transpiration. 



It is needless now to go into the many arguments which overthrew 

 the imbibition hj^pothesis. It is enough to say that the ingenuity 

 of the theory and the reputation of its elaborator and defender made 

 it survive an incredibl}^ long time despite the accumulation of crushing 

 evidence from structure and of clear inference from experiment. 



Of all investigators Böhm's attitude to the problem of the ascent 

 of sap is perhaps the most difficult to estimate. This is partly 

 because he changed his view, upholding first the air pressure theory 

 and. afterwards, with equal vigour championing capillarity as the 

 lifting force — and partly because his work influenced his con- 

 temporaries so little and received such scanty attention from them. 



B Ö h m 's own writings, w^liich are always polemical in tone and in 

 some places self-contradictory, were probably largely responsible for 



>) F. Elfving, Ueber die Wasserleitung im Holz. Bot. Ztg., 1882, p. 707. 



-) J. V es que, Eecherches sur le mouvement de la sève ascendante. Ann. d. 

 Sciences Nat. Bot., Ser. VI, 19, 1884, p. 188. 



") M. Scheit, Die Wasserbewegung im Holze. Bot. Ztg., 1884, p. 177. 



^) L. Errera, Une expérience sur l'ascension de la sève chez les plantes. 

 Compt. rend, de la Soc. roy. de bot. de Belgique, Bull. XXV, 2, 1886, p. 28. 



-'} E. Strasburger, Ueber den Bau und Verrichtungen der Leitungsbahnen 

 in den Pflanzen. Jena 1891. 



ö) H. H. Dix on and J. Joly, The Path of the Transpiration Current. Ann. 

 Bot.. Vol. IX, 1895, p. 404. 



