56 TRANSPIRATION AND ASCENT OF SAP ch. 



lumina immediately above the killed region. Even with- 

 out Weber and Janse's direct determinations it would be 

 hard to believe that the deposit of this coloured substance 

 in the walls and lumina of the tubes could be without 

 effect on their efficiency in transmitting water. 



From what has been said it will appear that even in 

 those cases where no visible stoppage has been found, we 

 have ample reason to suspect its presence. Furthermore, 

 in many cases, both where the gum-like clogging material 

 has been observed, and where it has not been found, the 

 bordering living cells develop tyloses, and so more or less 

 completely stop the flow of water in the tracheae. 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 in the 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 explic- 

 able 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. 



Contamination of the sap. It also seems certain 

 that some of the materials liberated into the transpiration 

 stream 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 reduc- 

 tion in their water supply. It actually, then, appears, 

 as Vesque puts it, that the leaves dry because they die. 1 



1 Here it may be pointed out that the leaves which fade after their sup- 

 porting branch has been killed by heat, fade in a different manner from 



