1 6 How Plants are wurifhed. [Book VIIL 



margin nly. This obfervadon in itfelf is not how- 

 ever decifive. For it may be fuppofed, that the liquor 

 flows more copioufly from the fuperior margin, be- 

 caufe the prefiure of the air is lefs upon it, than on 

 the inferior, and becaufe the liquor itfelf is difpofed to 

 fall downwards by its gravity, in the fame manner as 

 the Juccus communls. That I might put this matter 

 out of doubt, I placed the branch of a pine in a ho- 

 rizontal pofttion, and another branch I inverted, fb 

 that its branches were turned towards the earth. In 

 thefe fituations, I cut a portion of the bark and wood 

 from each, and in both inftances, the Juccus proprius 

 flowed only from thofe margins of the incifions which 

 were fartheft from the roots. Hence it appears clearly, 

 that the courfe of this juice, in its veffels, is never 

 from the roots towards the branches^ but always in the 

 contrary direction *.' 



M. Bonnet conceives that the nutrimental juices of 

 vegetables pafs during the day-time from the roots to 

 the trunk by the ligneous fibres, affifted by the air- 

 veffels, and are principally carried to the furface of the 

 leaves, where a copious perfpiration takes place. At 

 the approach of night the heat no longer ailing on the 

 leaves and the air contained in the air- veffels, the fap 

 returns towards the roots j at the fame time that the* 

 humidity condenfed on the inferior furfaces of the 

 leaves, which by their inequalities are bed fitted to 

 retain it, is abforbed and conveyed through the 

 branches to the trunk. In this manner he is of opi- 



* From the experiment above recited, it appears, that the flow of 

 the proper juice is not influenced in the fame degree, as that of the 

 fap, by an alteration in the pofture of the veffels fcom which it 

 itfues. To what caufe this is owing, does not clearly appear. 



nion 



