32 Scientific Proceedings, Royal Dublin Society. 



that some clogging material is drawn back or chokes tlie conduits leading 

 to the leaves in question. 



From these observations I feel compelled to assume that at least part of 

 the fading of leaves of steamed branches is due to the action of deleterious 

 or poisonous substances on their cells. Nor do I regard as a serious objection 

 to this view the fact that the action of these substances is not so rapid nor so 

 complete as that of a solution of copper chloride.' 



That the acceleration of the shrivelling is in many oases due to clogging 

 of the tracheal walls and to the stoppage of their lumina is shown by 

 direct microscopic observation. It is also indicated by "Weber's results, 

 who found that in some cases the simple reduction of the leaf area above 

 the steamed region was sufficient to restore for a short time turgescence 

 to the remaining leaves. In these cases presumably the water-supply, 

 reduced by coming through the partially clogged branch, together with the 

 water already in the brancli, was sufficient to restore turgescence temporarily 

 to the remaining leaves. This supply being quickly exhausted, they 

 withered finally, as the small supply coming through the clogged branch 

 was insufficient to make up for their loss by evaporation. 



The recovery of the upper leaves when supplied with water above the 

 steamed region is again a demonstration that the final stages of fading are 

 due to the increased resistance opposed to the water-supply by the clogging 

 materials in, or coming from, the steamed region. 



This increased resistance is also indicated by "Weber's observation' that, 

 even with a head of 62 cm. of mercury, appreciable amounts of water could 

 not be forced through about 12 cm. of the basal part of the heated branch. 



The experiments and observations just detailed show, I think, that there 

 is no reason to believe that vital actions in the stem are needed to assist the 

 transpiration current ; the anatomical relations of the cells of the stem to 

 the conducting tubes are also against the view that they can apply any 

 elevating force. The ingenious suggestions of Ewart'^ show the difficulties 

 a supporter of the vital theory is in on this account ; while in TJrsprung's* 

 diagram there seems no reason why the water forced into the conducting 

 tubes should not move downwards rather than upwards. 



Seeing, then, that there is no adequate reason for supposing the cells 

 of the stem to assist in the raising of the transpiration current, it is of the 

 greatest interest to find out if, to raise the water in the highest trees, the 



' TJrsprung, "Ueber die Ursache des Welkens." Beihefte z. Bot. Centi'albl., 1905. 

 * Weber, loc. cit. 



^Ewart, Eoy. Soc. Phil. Trans., B. 1905 and 1907. 

 . ■• Ureprung, Biolog. Centralbl. Bd. xxvii., 1907, p. 53. 



