lg Henry H. Dixon. 



part is enormously increased, is a piece of positive evidence in favour 

 of stoppage which cannot be put aside. He found that the lower 

 part (11"8 cm long) of the heated stem, including- both boundaries 

 between the dead and the living regions, refused to transmit water 

 under a head of 62 cm of mercury. Microscopic observation shows 

 that clogging is greatest in these border regions. The permeability 

 of the upper parts of the same stem was much greater, as was seen 

 from the fact that the upper 16"5 cms supporting the faded leaves 

 transmitted sufficient water under a head of 59 cm of water to render 

 the leaves turgescent once more. In several other experiments 

 Weber has shown that the resistance of the conducting wood is 

 enormously increased where the uninjured part borders on the heated 

 region. Janse') confirmed and extended Weber's observations of 

 this increase of resistance even when the temperature to which the 

 branch was exposed was as low as 60" — 64*^0. 



It is easy to show that the water traversing a piece of stem 

 killed with heat is contaminated in its passage. 



Thus if distilled water is forced through a piece of a stem 

 freshly cut from a tree, it is transmitted without sensible colouration 

 as long as morbid changes in the stem do not take place. But if 

 the same stem is surrounded with steam so that its cells are killed, 

 the water which emerges is no longer colourless, but is tinged, more 

 or less deeply, with brown.^) \Mien attached to the tree the killed 

 piece must contaminate the rising transpiration stream in the same way. 

 The record of this contamination first appears in the walls of the 

 tracheae of the leaves which become stained, then in their lumina 

 which become filled with the coloured fluid concentrated by evapora- 

 tion. This substance is finally caught in the walls of the conducting 

 tubes of the stem and forms plugs filling their lumina immediately above 

 the killed region. Even without Weber and Janse's direct deter- 

 minations it would be hard to believe that the deposit of this coloured 

 substance in the walls and lumina of the tubes could be without eifect 

 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. 



1) J. M. Jan se, Jahrb. f. Wiss. Bot., 18. 1887. 



2) H. H. Dixon, Vitality and the Transmission of Water through the Stems 

 of Plants. Proc. Roy. Dubl. Soc, XII, 3, 1909, and Notes from the Botanical School 

 of Trinity College, Dublin, Vol. II, 1. 1909. 



