116 Alfred J. Eivart : 



until it is answered it is not permissible to assume that the results 

 obtained in glass tubes with stationary columns of water can be 

 directly applied to the flowing columns of water, surrounded by the 

 rough, water impregnated walls of the, wood vessels, which are also 

 readily permeable to air under pressure. 



In a second paper, experiments were conducted on entire trees to 

 determine whether any of the high tensions postulated in the water 

 tension theory in the ascent of sap, could be detected in the wood of 

 actively transpiring trees. The results obtained were in the negative, 

 but, as pointed out by Dixon, the ordinary manometer experiments 

 are unable to provide against the existence of air cavities in the wood 

 tissue, so that the pressure exhibited by a manometer might be con- 

 siderably less than that actually existing in the cavities of the wood 

 vessels themselves. In any case this very fact makes it difficult to see 

 how a high tension could be maintained for any length of time in a 

 w^ater column contained in a tube w^hose walls were saturated with 

 water, and which bordered externally upon air spaces. Tlie appear- 

 ance of the minutest bubble of air in such a column of water would 

 immediately cause its tension to be reduced to some fraction of an 

 atmosphere. Actual observations, which have been confirmed by 

 more than one observer, have shown that the wood vessels in the 

 functioning wood of actively transpiring plants do actually contain 

 bubbles of air, and hence cannot possibly transmit any tension exceed- 

 ing an atmosphere. 



In the same paper an account was given of an experiment with an 

 entire tree, carried out on the same lines as those by Strasburger — 

 namely, by cutting an entire tree at its base, and allowing first a 

 poisonous and then a coloured solution to be drawn up the trunk of 

 the tree. TTie experiments showed that there was a distinct tendency 

 on the part of the sap to avoid the parts of the wood which had been 

 killed l)y poison, and to flow in the older parts of the wood to which 

 less poison had penetrated, but in which the flow is usually least active 

 under normal conditions. 



Apparently this pointed to tlie necessity of the existence of living 

 wood cells to maintain the function of the wood vessels as conducting 

 chambers, even for short lengths of time, and this would tend to show 

 thiit the water tension theory onlv afforded a paiiial explanation of 

 the ascent of water in tall trees. It was, however, obviously .advisable 

 to complete such observations l)v experiments canicd out on tlie 

 tallest trees available of 200 to :^I10 feet in lieight. The initial diffi- 

 culty lay, however, firstly in the comi»arative inaccessilulity of such 

 trees for scientific experiments, and secondly in the difficulty and cost 

 of caiTving out the ri'(|uii-('d mani]uilations. whii'h would include very 



1 I'liil Trails. Koy. Soc. London, 11, vol. 100, y. 341. 



