Dixon — Vitality and Transmission of Water through Plants, 31 



only appears after shrivelling, and occurs in irregular patches. The veins do 

 not change colour, and the walls of the trachese do not appear coloured in 

 transverse sections. The first appearance of colour-change is when the cell- 

 contents of the mesophyll and parenchyma of the veins colour brown after 

 death. 



This contrast of behaviour appears to me inconsistent with the view that 

 the leaves in both cases fade from precisely the same cause, viz., simply and 

 solely failure of water-supply. The fact that the mesophyll of the leaves of 

 the killed branch discolours before shrivelling, while, when the water-supply 

 only is interfered with, shrivelling precedes the discoloration, appears to 

 support Yesque's conclusion that in the previous case the leaves dry because 

 they die, while in the latter they die because they dry.' But there is some- 

 thing more than this. While the discoloration is proceeding in the 

 mesophyll of the leaf the walls of the conducting tubes of the veins are 

 becoming discoloured, and finally their lumina become clioked with a trans- 

 parent coloured material. The experiment described on page 26 indicates 

 the origin of this material. Distilled water urged through a branch 

 jacketed with steam issues as a coloured and more or less viscid fluid. The 

 transpiration current also sweeps this clogging material before it; and, 

 being deposited iu the walls and lumina of the upper conducting tubes, 

 it reduces their power of transmitting an adequate water-supply upwards. 

 From this it would appear that the initial stages of the fading were caused 

 by the poisoning of the mesophyll cells, while the final stages, withering and 

 drying, were accelerated by the clogging of the walls, and stoppage of the 

 lumina of the supply-tubes. 



In a previous paper^ I have described how it is possible to cause the 

 leaves of a branch to fade by supplying it with water which has passed 

 through a killed piece of stem, although its direct supply of water from the 

 root is not interfered with. Since then 1 have confirmed these observations ; 

 and I have found that the leaves on a lateral branch, arising close below 

 the killed region of a stem, may often exhibit the kind of fading which I 

 have just described as characteristic of the leaves on killed branches. As in 

 the previous cases the direct connexion between these leaves and the 

 roots had not been interfered with, but their original water-supply coming 

 up through the uninjured parts of the stem was left to them intact. The 

 only cause I can suspect as being responsible for their succumbing is the 

 passage backwards into their transpiration -stream of some harmful material 

 from the dead region above the base of the lateral branch. It may also be 



1 Vesque, Compt. Rend., 1885. ■ Eoy. Dublin Soc. Proc, Tol. x., 1905, p. 48. 



