Transpiration and the Ascent of Sap. \g 



The occurrence of tyloses has been recorded in a large number of 

 cases. 



Thus the fading- and drying of the leaves above the killed region 

 gives no support to the idea, that the lack of water from which the 

 leaves suffer is due to the removal of vital forces which are required 

 to raise the water-supply, but it rather indicates a great increase 

 of resistance of the stem, due in part to the stoppage of the lumina 

 and to the clogging of the walls of the transmitting tracheae. Janse's 

 and Ursprung's observation, that the greater the length of the 

 killed portion the more rapid the fading, is quite explicable on this 

 view, as it is natural that a greater amount of the clogging material 

 would be set free by the larger number of killed cells. 



It also seems probable that some of the materials liberated by 

 the killed cells act deleteriously on the cells in the leaf and bring 

 about morbid changes in them so that in many cases these cells 

 actually lose their turgor and die even before they suffer severely 

 from the reduction in their water supply. Then it actually sometimes ' 

 appears, as Vesque puts it, that the leaves dry because they die. 

 The author has succeeded in showing the effect of these hurtful sub- 

 stances even on leaves whose normal water supply is not diminished. 

 To carry out this observation ') one proceeds as follows : One arm of 

 a forked branch, after being stripped of its leaves and deprived of its 

 apex, while still attached to the tree is surrounded with steam or 

 immersed in hot water. Its cut distal end is then supplied with clean 

 water. Transpiration from the leaves of the other arm is thus 

 supplied with water coming from the main stem and also with water 

 passing back through the killed fork. The leaves on the living arm 

 usually soon show appearances resembling the early stages of fading 

 occurring on branches which are supplied solely through a killed 

 stem. In these cases, although the water supply has been actually 

 increased, fading has supervened from the poisoning of the additional 

 supply passing through the killed wood. This (viz., poisoning) is 

 another cause to account for the fading of the leaves without 

 supposing that it is the loss of vitality of the stem which reduces 

 their supply. 



In another form the vital hypothesis has been lately supported 

 by E w a r t.'-) Adopting J a n s e 's method of determining the resistance 

 opposed to the transpiration current, Ewart obtained results which 

 indicate that, in order to move water in stems of plants at the 



') H. H. Dixon, Note on the Supply of Water to Leaves on a dead Branch. 

 Proc. Roy. Dnbl. Soc, 1905, pp. 7 et seq. 



2) A. J. Ewart, The Ascent of Water in Trees. Phil. Trans, of Roy. Soc. 

 London, Ser. B Vol. 198 pp. 41—85. 1905. 



2* 



