48 



CHAPTER IX. 



OF THE SAP, AND INSENSIBLE PERSPIRATION. 



1 HE sap of trees, as has been mentioned in the last 

 chapter, may be obtained by wounding a stem or branch 

 in spring, just before the buds open, or in the end of 

 autumn, though less copiously, after a slight frost; yet 

 not during the frost. In the Palm-trees of hot countries, 

 it is said to flow from a wound at any time of the year. 

 It has always been observed to flow from the young 

 wood or alburnum of our trees, not from the bark ; 

 which agrees with Mr. Knight's theory. 



A common branch of the Vine cut through will yield 

 about a pint of this fluid in the course of twenty-four 

 hours. The Birch, Betula alba, affords plenty of sap ; 

 some other trees yield but a small quantity. It flows 

 equally upward and downward from a wound, at least 

 proportionably to the quantity of stem or branch in 

 either direction to supply it. Some authors have as- 

 serted that in the heat of the day it flows most from the 

 lower part of a wound, and in the cool of the evening 

 from the upper ; hence they concluded it was ascending 

 during the first period, and descending in the latter. If 

 the fact be true, some other solution must be sought; 

 nor would it be difficult to invent a theory upon this 

 subject: but we rather prefer the investigation of truth 

 on more solid foundations. 



