Transpiration and the Ascent of Sap. 21 



water column below as much as it would raise that above it, and 

 such a substance once introduced would render the tube unavailable 

 for tlie rise of other columns until it were completely removed. The 

 removal of an unwetable substance would be attended with difficulties; 

 for we could not assume the exudation of an emulsifying- substance 

 into the tracheae, as this, when once diffused into the columns, would 

 nullify the whole action. His third suggestion also is open to the 

 objection, tliat the increased surface tension drawing up one column 

 will drag- down the one above, and a diminished tension will lower 

 one if it allows another to rise. 



These hypotheses are put forward only in a tentative way by 

 their author and indeed they are beset by so many physical diffi- 

 culties that no one could think that any one of them actually 

 represents the state of affairs in the conducting tracts. Nevertheless 

 they are of importance as emphasising the fact that up to the present 

 it has been impossible to show liow 'pumping actions' in the cells 

 adjoining the tracheal tubes can assist in elevating the water in the 

 tracheae. 



Not only has it been found impossible up to the present to 

 devise a scheme whereby the cells of the stem should exert a pro- 

 pulsive action on the water in the tracheal tubes, but also experiment 

 has totally failed to give indications of such a force. Ewart^) 

 sought to demonstrate it by using a long piece of Wistaria stem 

 connecting two vessels of water, but in no case did he obtain 

 unequivocal evidence of the existence of a pumping action in the 

 stem. The author also carefully compared the downward rate of 

 transmission of water in a branch before and after death, but in 

 no case was the resistance to downward movement reduced by the 

 killing of the stem. The methods used would detect -) a comparatively 

 small force if such existed, and it is to be noted that the forces 

 which E wart's estimates of resistance necessitate would be very 

 considerable. 



It is however satisfactory to know that the estimates of resistance, 

 which seemed to necessitate these improbable forces, are excessive^), 

 and although we have no precise data for the resistance and the 

 amount of transpiration of the highest trees, there is no reason to 

 assume that the resistance to their transpiration stream is anything 



') A. .J. Ewart, The Ascent of Water in Trees [Second Paper]. Phil. Trans. 

 Roy. Soc. Lond., Ser. B Vol. 199, 1908, p. 379. 



^) fi. 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. 



') Idem, On the Transpiration Current in Plants. Proc. Roy. Soc. Loud., B, 

 Vol. 79. 1907. 



