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The Ascent of Water in Trees : (Second Paper). 



By A. J. Ewart, D.Sc. Lond., Professor of Botany, University of 



Melbourne. 



(Communicated by Francis Darwin, For. Sec. K.S. Keceived April 18, — 



Kead May 9, 1907.) 



(Abstract.) 



The physical conditions in the vessels interpose no obstacle to the exertion 

 of a pumping action on their contents controlled by the medullary ray cells. 

 There are many theoretical ways in which surface tension action could be 

 brought into play in this manner, but no practical proof of the existence of 

 any such action in the wood vessels has as yet been obtained. Similarly, 

 the bleeding of roots and stems affords no absolute proof of the existence of 

 a vital pumping action in them, since all the observed cases can be explained 

 as the result of osmotic action coupled with a reabsorption of the osmotic 

 materials from the ascending stream, and their surface adsorption by the walls 

 of the vessels. Instances are given in which an apparent pumping action 

 proved to be of this character. 



Nevertheless, the experiments detailed or quoted in the above paper all 

 tend to show that the continuous ascent of water is only possible in living 

 wood, and that the power of conduction is rapidly lost on death, without any 

 mechanical blocking of the vessels being necessarily responsible for the 

 change. Hence we are forced to conclude that the living cells in tall trees 

 •continually restore the conditions for the ascent of water wherever these are 

 affected by the excessive emptying of the vessels, and decrease the resistance 

 to flow, as far as possible, by maintaining continuous water columns in parts 

 .at least of the wood. So long as these are present ah initio, a pumping 

 action only becomes necessary in trees over 20 to 50 metres in height, but 

 suspended columns cannot be maintained for any length of time in the vessels 

 of tall trees without the aid of the living cells of the wood. 



The energy required to pump water upwards in the tallest trees represents 

 only a small fraction of that produced by the daily photosynthetic assimi- 

 lation, and it is the feeble character and diffuseness of the pumping action 

 which renders it so difficult to demonstrate practically. 



Contrary to Strasburger's statement, no vessels appear to run as open 

 channels from end to end of any tree, the longest vessel observed being 

 564 cm. as measured by the length of stem in which it ran ( Wistaria). In 

 this plant, however, and others also, owing to the irregular course followed by 



