Dixon — VitaliUj and Transmission of Water through Plants, 29 



and the washing away of mechanical obstructions by the stream under hiffh 

 pressure, and partly to the small rise in temperature from 11'8° to 12'0°, 

 which would perceptibly diminish the viscosity of the water. Inasmuch as the 

 observed rise is as great in the case of the living branch as in that which is 

 killed during the observations, it follows that there were no vital actions in 

 either retarding the transmission. 



J2-f T,mf o/jiuJccl 



The rapid or ultimate fading of the leaves after the death of the support- 

 ing stem, or some portion of it, has been often used as an argument in favour 

 of the view that vital processes in the stem participate to a large extent in 

 the lifting of the transpiration stream.' This interpretation of the observation 

 is arbitrary, and, before accepting it, we must assure ourselves that no other 

 change, introduced by the method of killing the stem, is responsible for the 

 fading of the leaves, but that this latter is simply due to the inability of the 

 leaves to obtain water when the cells of the stem cease to assist in raising 

 water to them. A priori it would appear that the observation would receive 

 a more natural explanation if, instead of assuming that the lifting force is 

 reduced, we suppose that the killing of the cells of the stem allows substances 

 to pass into or form in the water-capillaries which may render the walls of the 

 trachese less permeable to water, or which may act injuriously on the cells of 

 the leaves and cause them to fade.- Jause's^ and XJrsprung's^ observation 

 that the greater the length of the stem killed below the leaves, the more rapid 



' Janse : Pringsheim, Jahrbuch, 1887. Ursprung : Beihefte, Bot. Centralbl,, 1904, and Jahrb. f. 

 ■wiss. Bot., 1906 and 1907. 



2 Webev, Ber. d. deutsch. Bot. Ges. 1885. ^ Janse, he. eit., 1887. * Ursprung, loc. eU., 1904. 



SCIENT. PBGC. R.D.S., VOL. XII., NO. III. F 



