iv.] VARIOUS THEORIES, &c. 77 



theses as follows. Unger, Pfeffer and Sachs may be 

 regarded as the exponents of the imbibition theory ; 

 while Hartig, Naegeli and Schwendener, and Boehm 

 are the chief adherents to the view that the water 

 ascends in the cavities or lumina of the vessels and 

 tracheides the only general point of agreement 

 between the botanists last mentioned, however, being 

 that the lumina constitute the route of the water, for 

 they differ greatly in details. 



Elfving set himself the task of subjecting the two 

 rival hypotheses to experimental tests. He employed 

 the wood of the yew, unless otherwise specified. 

 Having verified Th. Hartig's experiment, he attached 

 a piece of yew branch, a few centimeters long and 

 about one centimeter thick, to the end of a piece of 

 caoutchouc-tubing, and showed that very gentle 

 blowing and sucking through the tube caused the 

 alternate expulsion and withdrawal of water, at the 

 cut face of the alburnum. 



He then showed that the yew-wood is readily 

 permeable to all kinds of fluids, very little pressure 

 being needed to drive the following in succession 

 through the same piece, and in the order given 

 viz. water, alcohol, benzol, alcohol, water, dilute am- 

 monia, water, dilute acetic acid, water, alcohol, &c. 

 Under great pressure he could even drive a solution 



