207 



XII. 



NOTE ON THE SPREAD OF MORBID CHANGES THROUGH 

 PLANTS FROM BRANCHES KILLED BY HEAT. 



By HENEY H. DIXON, Sc.D., F.R.S., Univ. Professor of Botany, 

 Trinity College, Dublin. 



[Read December 16, 1913. Published Februaky 3, 1914.] 



Some years ago' I showed that, if a branch of a plant is killed by heat, and 

 the rest of the plant is then supplied with water tlirough this branch, some 

 of the leaves on tlie uninjured branches may become injuriously affected, 

 although there is no other interference with their direct supply from the 

 roots. This result may be explained as due to the formation of poisonous 

 bodies in tlie lieated branch and their transference in the water-supply to 

 tlie other branches. 



The fact that the withering of the leaves in these cases is due to the 

 contamination of the water-supply demonstrates the invalidity of the con- 

 clusion that the fading of leaves above a steamed region of a stem must be 

 attributed to the cutting-off of the water-supply by interfering witli the vital 

 actions of the wood parenchyma and medullary ray cells in the stem. 



Shortly after completing the experiments quoted above, I made additional 

 experiments which seemed to me to bear out my previous conclusion in a still 

 more striking manner. However, as the point seemed adequately established 

 by former work,- the record of these experiments was put aside iu favour of 

 more pressing investigations. 



Quite recently attention has been again directed to the subject by 

 Ursprung.^ He reiterates the earlier explanation, and states tliat he could 

 observe no poisoning effects below the killed region such as were observed iu 

 my experiments. This negative observation in itself would, of course, mean 

 nothing unless it were quite certain that a sti-eam of water passed from the 

 killed region to the healthy leaves, and so that there was an opportunity for the 

 dissolved substances in the killed region to pass into the other parts of the 

 plant. The partial restitution of turgescence observed by Ursprung, if wilting 

 liad not gone too far when the dead branches were supplied witli water under 



1 Eoy. Dublin Soc. Proc, vol. x, 1905, p. 48. 



- J. B. Overton: Transpiration and Sap-flow. Bot. Gaz., 1911, pp. 28 and 102. These papers 

 contain a summary of the discussion, together with new and convincing experimental work. 



^ A. Ursprung : Zur Frage nach der Beteiligung lebeuder Zellen am Saftsteigen. Beihefte znm 

 Botan. Centralblatt. Bd. xxviii (1912), Abt. I, p. 311. 



SCIENT. PROC. K.D.S., VOL. XIV., KG. XII. 2 I 



mt li 1914 



