ON PRUNING. 



151 



warm wax to the wounded part ; I have, however, 

 sometimes found, that soot, or charcoal-dust ground 



by the horizontal direction of their branches, is fully sufficient 

 to propel the sap, without the undulatory motion that I have 

 mentioned. 



" I beg leave to observe, that these observations are only 

 intended to convey a general idea of the motion of the sap. It 

 varies according to the temperature of the weather. The air 

 is seldom one moment alike. The sap must, therefore, some- 

 times move quick and sometimes slow. It may rise and fall many 

 times in a day. Sudden heats push it upward, sudden colds 

 make it fall. Thus the juices are blended, and the secretions 

 forwarded. 



" The manner that the nutritive juices of the earth and 

 atmosphere are conveyed into the sap-vessels remains to be 

 described. And this makes a necessary part of our present 

 argument, as it may assist us in finding out and explaining the 

 diseases of plants from the variations of the weather. 



" The outer bark, which covers every external part of a 

 vegetable, as well below as above the surface, is full of perspir- 

 atory or absorbent holes. The vessels of this bark, being 

 endowed with the power inherent in capillary tubes, draw up the 

 moisture that is applied to their surface. From them it is com- 

 mitted to the vessels of the inner bark. After receiving some 

 degree of melioration, the sap is delivered to the blea. From 

 the blea it passes, by anastomising canals, to the* vascular series. 

 From thence to the wood, or flesh, where it receives its last 

 concoction. 



" The nutritive particles, being separated by the mechanism 

 of these numerous canals, are applied towards the fructification 

 and increase of the plant, while the watery and excrementitious 

 parts are carried expeditiously to the leaves, where they are 

 perspired off in the form of vapour. It is evident, however, that, 

 as water contains but few particles that are fit for nourishment, 

 it was necessary that plants should have the power of imbibing 



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