PREFACE 



The Ascent of Sap is a problem of old standing. As 

 was the case with several other biological problems, a 

 peculiar alternation may be observed in the hypotheses 

 formed to explain the phenomena. At first the process 

 of the ascent of water in trees was, almost without serious 

 thought, assigned to the vital activities of the plant, 

 and, in general with other vital processes, put outside 

 the domain of physical investigation. Later, advances 

 in physics and chemistry, introducing rationality into 

 observation, emboldened hardy spirits to assign the 

 whole process to various physical forces or to combinations 

 of them. Doubtless these philosophers somewhat trans- 

 gressed legitimate deduction, and their daring met its 

 punishment in the overthrow of their successive physical 

 theories. It was then again the turn of the Vitalists, 

 and during the latter part of the last century they erqoyed 

 their heyday of dogma. However, the leaven of ration- 

 ality still worked on in even their theories, and several 

 physiologists must have felt what Strasburger stated, 

 that the physical forces developed in, and the physical 

 configuration exhibited by, the water tracts would supply 

 a complete explanation when properly understood. 



In the present monograph, an account is given of a 



