120 PLANT PHYSIOLOGY 



skTs hypothesis, on the face of it much more reasonable, 

 /but not directly supported by experiment,(that living cells 

 j adjoining ducts and tracheids exert a pumping action^ 

 By dissolving poisonous substances in the water to be ab- 

 sorbed by the plants selected for examination, Strasburger* 

 attempted to demonstrate that living cells are not con- 

 cerned. Sap-pressure (see pp. 127, 131) was for a time con- 

 sidered the propelling force, but the absence of sap-pressure 

 at the times when water is most needed is sufficient evidence 

 against this notion. The varying pressure of the gases 

 within the body of the plant was supposed to be the answer 

 to the question, until it was shown that gas-pressures do 

 not vary enough and rapidly enough. Then came the era 

 of Jamin's chains, when the ducts and tracheids, found to 

 contain alternating columns of air and water, were supposed 

 to furnish the tubes through which these pass. Schwen- 

 dener,f Dixon and Joly,J and Askenasy have contributed 

 much to a knowledge of the physical qualities of such 

 chains, but no one has succeeded in proving that they have 

 much if anything to do with water-transfer. Eepeatedly 

 botanists have returned, from lack of anything better, to 

 the idea that capillarity conveys the water through the 

 vascular system ; but this notion is inadequate because the 

 vascular system of many plants is composed of tubes so 

 small and so short that, although the capillary force is 

 great, the resistances are also so great that a sufficiently 

 rapid transfer by this means alone is inconceivable. 



From all the contradictory views this much may be ex- 

 tracted as proved. The water, which certainly permeates 

 the walls of the elements composing the vascular system, is 

 also contained in the cavities and passes through the cavi- 

 ties in the direction of strongest attraction, conversely of 

 least resistance. That the water does actually pass into 



* Strasburger, E. Bau und Verrichtung der Leitungsbahnen, 1891. 

 tJber das Saftsteigen, 1893. 



f Schwendener, S. In Sitzungsberichte der Berliner Akademie, from 1886 on. 



J Dixon and Joly. On the ascent of sap. Annals of Botany, 1894. The 

 Path of the transpiration current. Ibid., 1895. 



Askenasy. IJber das Saftsteigen. Verhandlung d. naturh. Vereins in 

 Heidelberg, 1895. 



