30 Scientific Proceedings^ Royal Dublin Society. 



is the fading, seems to me to favour this explanation. Failure to ohserve 

 plugging of the lumina of the tracheae in the killed region is no disproof 

 that the partitions, which the stream normally traverses, are rendered less 

 permeable, and that more complete plugging occurs higher up in the path 

 of the water.' Both these surmises have already received experimental 

 support,- and are further confirmed by some observations to be described 

 later. Furthermore, the possibility that the leaf-cells fade from poisoning 

 must be admitted until evidence to the contrary is forthcoming.' 



When a length of a branch is jacketed with steam so as to kill it, the 

 fading of the leaves above which supervenes is not identical with that which 

 occurs when leaves are deprived of adequate water-supply. To quote an 

 example: a branch of Populus, about 160 cm. long, carrying about 20 leaves, 

 was jacketed for a length of 85 cm. with steam for 10 min. On the 

 following day the older leaves had lost their fresh lustre ; and on the fourth 

 day they had considerably changed. This change consisted in a general 

 dullness of colour over the whole leaf. Later the margin of the leaf became 

 dark ; and this darkness gradually invaded the leaf between the veins, leaving 

 a green border along the veins. The darkened margin subsequently dried 

 and shrivelled, while the small terminal branches, the petioles, and the green 

 parts of the leaf round the veins remained fairly turgescent. As the change 

 proceeded in the mesophyll, the veins became coloured pink, and finally red- 

 brown. This coloration was particularly noticeable when the leaves were 

 viewed by transmitted light. Microscopic examination of the leaves showed 

 that protoplasm of the mesophyll cells in the dark areas had contracted, and 

 the cells were no longer turgescent, and the chlorophyll corpuscles had 

 become discoloured, and bore a dirty brown-green tinge. During even the 

 early stages of fading, sections of the veins revealed a pink coloration in the 

 walls of the trachese, while the contents of the wood parenchyma were 

 uncoloured. Later on the lumina of the trachese became filled with a pinkish 

 brown material which ultimately seemed to choke the tubes. Shrivelling and 

 withering of the leaf, except at the edges, did not occur till after these 

 changes were complete. I have traced changes similar to these in the leaves 

 of Tilia microphylla, Syringa vulgaris, Salix viminalis, and Acer Pseudo-Platanus, 

 when the supporting stems had been steamed. 



On the other hand, wheu leaves fade simply from an insufficient water- 

 supply, e.g. on a branch severed from a tree, shrivelling comes on while they 

 are still green ; the smaller branches and the petioles also shrivel. Blackening 



'Weber, he. cit., observed obstructions either in the form of a clogging material or of tyloses 

 in the trachese, where the killed region adjoins the living. 



2 Dixon, Koy. Dublin Soc. Proc, vol. x., 1905, p. 48. = Dixon, he. cit. 



