SECT. III. CIRCULATION OF JUICES. 23Q 



by the fact that an inverted plant grows. It was 

 also added that a malignant humour has been some- 

 times found to pervade the whole of the vascular 

 system ; which could not have happened, as it was 

 thought, except upon the supposition of a circulation 

 of fluids. But it is also known that diseases of the 

 trunk do not always affect the root ; and that if a 

 tree diseased in the trunk or branches is cut down 

 to the root, it will send up new shoots as sound and 

 vigorous as at first. Finally, it was said that para- 

 sitical plants are injurious to the tree on which 

 they grow, by throwing into the circulation some 

 noxious principle. But it is not proved that para- 

 sitical plants are always injurious to the tree on 

 which they grow; and if they are so sometimes, 

 the effect may be very well accounted for by attri- 

 buting it to the privation of a part of its due nou- 

 rishment, rather than by the introducing of some 

 noxious principle into an assumed circulation. 



Such are the principal arguments that were ad- 

 vanced by the earlier phytologists in support of the 

 circulation of the sap, as stated and refuted by Du 

 Hamel,* who, while he admits the ascent of the sap, 

 and descent of the proper juice, each in peculiar and 

 appropriate vessels, does not however admit the doc- 

 trine of a circulation ; which seems, about the mid- 

 dle of the last century, to have fallen into disrepute. 

 For Hales, who contended for an alternate ascent and 

 descent of fluids in the day and night, and in the same 

 vessels, or for a sort of vibratory motion as he also 



* Phys. des Arbres, liv, v. chap. ii. 



