ON P R-U N I N G. 
141 
or charcoal diiH: ground very fine, and mixed with foft foap to 
the confiftence of pafte, has proved more efficacious. 
It 
“ veflels are uiddenly ftraitened. The contained juices confequently receive 
“ reiterated inipulfes, fimilar to what happens to the blood of animals from 
“ the contradfion of the heart. Thi^bfervation may affift us in inveftigating 
“ the vegetable ceconomy, fo far as it regards the management of fruit-trees, 
“ and, probably, may be extended throughout the whole fyftem of gardening, 
“ planting, and farming. 
“ It may be objedled, that trees fixed to the wall do, notvvithftanding, carry 
“ their fap to the extreme branches ; but it fliould be confidered, that the 
“ warmth of their fituation, aflifted by the horizontal diredfion of their 
“ branches, is fully fufhcient to propel the fap, without the undulatory motion 
“ that I have mentioned. 
“ I beg leave to obferve, that thefe obfervations are only intended to convey 
“ a general idea of the motion of the fap. It varies according to the tempera- 
“ ture of the weather. The air is feldom one moment alike. The fap muff, 
“ therefore, fometimes move quick and fometimes flow. It may rife and 
“ fall many times in a day. Sudden heats pulh it upward, fudden colds make 
“ it fall. Thus the juices are blended, and the fecretions forwarded. 
“ The manner that the nutritive juices of the earth and atmofphere are conveyed 
“ into the fap-veiTels, remains to be defcribed. And this makes a necefiary 
“ part of our prefent argument, as it may alTifl us in finding out and ex- 
“ plaining the difeafes of plants from he variations of the weather. 
“ The outer bark, which covers every external part of a vegetable, as well be- 
“ low as above the furface, is full of perfpiratcry, or abforbent holes. The 
“ veflels of this bark, being endowed with the power inherent in capillary 
“ tubes, draw up the moiflure tliat is applied to their furface. From them 
“ it is committed to the veflels of the inner bark. After receiving feme de- 
