192 HISTORY OF BOTANY 



I need not mention more than one critical experiment 

 which aimed at disproving the vitahstic theory, viz. 

 that of Strasburger, who showed that stems more than 

 ten metres in height still continued to draw up water 

 after all the Hving cells in it had been killed by exposure 

 to a temperature of 90° C. 



In 1894 an entirely new explanation of the puzzhng 

 phenomenon was put forward by Dixon and Joly, and 

 in the following year by Askenasy. The theory is based 

 on the cohesion of water molecules or the property 

 possessed by water of resisting tensile stress, so that a 

 column of hquid might be hfted bodily up a narrow tube ; 

 as Dixon puts it, " the water in the conducting tracts 

 of high trees hangs there by virtue of its cohesion." 



As I am not attempting to lecture to you on plant 

 physiology but only on the historical growth of our 

 knowledge of botanical problems, you will not expect 

 me to recount the experimental evidence brought forward 

 by Dixon and Joly in support of their hypothesis. That 

 you must read for yourselves, and perhaps most con- 

 veniently in Dixon's monograph on the subject published 

 in 19 14. I will content myself with quoting the summary 

 the author gives in his concluding chapter. 



" The transpiration stream is raised by secretory 

 actions taking place in the leaf cells, or by evaporation 

 or capillarity (imbibition) at their surfaces drawing 

 water from the tracheae. The state of saturation 

 surrounding these cells determines which of these agencies 

 is effective. 



"The configuration, physical properties, and structure 

 of the wood render the conducting tracts of plants highly 

 inefficient if regarded as a system for conveying water 

 urged upwards by pressure or drawn upwards in the 

 substance of the woody walls. The distribution of 

 living cells in these tracts is such that their actions 

 cannot account for the rise of water observed, and there 

 is no reason to beheve that the elimination of these 



