I. 



PAPERS OF 'THE PHYSICAL CLASS. 



I. E x P E R I M E N T s on the Motion of the SAP in TREES. By 

 JOHN WALKER, D.D. M. D. F. R. S. EDIN. and Regius Pro- 

 fejjbr of Natural Hi/lory in the Univerjity of EDINBURGH. 



\Readby the Author , Dec. 8. 1783, and Jan. 5. 1785.] 



THE difcovery of the circulation of the blood in animals was 

 foon followed by conjectures concerning the exiftence of 

 a like circulation in the fap of vegetables. Thefe conjectures 

 gave rife to the firft improvements in vegetable phyfiology, 

 which may be dated from the appearance of a fet of queries 

 concerning the motion of the juices of plants, publimed in 

 the Philofophical Tranfactions, anno 1668. Thefe queries im- 

 mediately engaged Dr BEALE and Dr TONGE, and afterwards 

 Dr GREW, Mr WILLOUGHBY, Mr RAY, and Dr LISTER, to en- 

 ter upon the fubjec*l ; who, in a great variety of obfervations, 

 ftruck out the firft confiderable difcoveries in the vegetable ceco- 

 nomy. 



THEIR obfervations, however, fell fhort either of proving 

 or difproving a circulation of the fap : Nor have the difcoveries 

 of a variety of Philofophers, fince that time, been able to demon- 

 ftrate either fide of the queftion ; which ftill remains a contro- 

 verted and undetermined point in the hiflory of vegetation. 



To trace the progrefs of the fap, in the way of experiment, 

 feems to be the only method by which we can expect to arrive 



at 



