LECTURE IX 



THE ASCENT OF SAP 



EVER since the time of Hales's classic experiment on 

 " ringing/' physiologists have been unanimous irt believing 

 that the path of ascent of sap in plants is by way of the 

 xylem, but how the sap is raised to the top of a lofty 

 tree is a problem on which the most divergent views have 

 been expressed. Hales's experiment, as one of our most 

 recent authorities on the subject has rightly pointed out, 

 was not aimed at determining the path of ascent, but 

 at showing that there did not exist in plants any regular 

 circulation such as occurs in animals. Hales himself 

 thought that ascent took place in the wood vessels in 

 virtue of capillarity, while Christian Wolff believed that 

 expansion of air co-operated in the upward movement. 

 Both these agencies were, however, soon seen to be 

 inadequate to explain the ascent beyond a certain height, 

 nor did any greater success attend the comparison of 

 the conditions existing in the wood with a Jamin's chain 

 of water columns and air bubbles. 



The first of the newer attempts to explain the move- 

 ment on physical principles was that made by Sachs, 

 in 1879, in a paper on " The Porosity of Wood/' and, in 

 1882, in his Lectures on the Physiology of Plants. " The 

 wood/' he says, " owes its significance as the organ for con- 

 ducting water to a series of most remarkable properties, 

 which are found in no other natural body. It depends 

 not upon a phenomenon of capillarity, but upon imbibition 

 and swelling. The one point of special importance to be 



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