238 Britain's Heritage of Science 



be mentioned here because of his profoundly important 

 discovery in connexion with the function of leaves. It was 

 the chemist Joseph Priestley (1733-1804), who, while working 

 on the investigation of the air, states : "I have been so 

 happy as by accident to have hit upon a method of restoring 

 air which has been injured by the burning of candles, and 

 I have discovered at least one restorative which nature 

 employs for this purpose. It is vegetation." He records 

 in 1778 that the green deposit in some vessels which he was 

 using for his experiments gave off very " pure air," and 

 discovered that this exhalation was given off when the algae, 

 as they proved to be, were exposed to sunlight. 



Thomas Andrew Knight (1759-1838) was the only out- 

 standing physiologist between Hales and the rise of the modern 

 school, and even he was more prominent as a horticulturist 

 than as a physiologist. He was educated at Balliol College, 

 Oxford, and, being in the possession of ample means, settled 

 first in Herefordshire and later at Downton, where he resided 

 until his death. He made the acquaintance of Sir Joseph 

 Banks, who was at that time seeking, on behalf of the Board 

 of Agriculture, certain correspondents who would answer 

 questions relating to agriculture in their several districts. 



Knight was the second President of the Horticultural 

 Society, which had been founded in 1804. He was elected 

 in 1810, and occupied the Presidential Chair until his death. 



His physiological investigations began with enquiries as 

 to the circulation of sap, and one of the methods of his 

 investigations was ringing the trees. He failed, however, 

 to appreciate the part that the leaf plays in nutrition, and 

 that the " function of the sap is to supply nutritive materials 

 to the various tissues and to circulate the manufactured 

 products of the leaf." 



But, as Professor Green reminds us, Knight's work on 

 the ascent and descent of sap " did much that was not only 

 instructive for the time," but " was destined to remain with 

 little modification among the fundamental facts of science." 

 He made certain anatomical discoveries in connexion with 

 these physiological experiments, and he incidentally investi- 

 gated the transpiration or, as it was then called, " the 

 perspiration," of the leaf, and showed that it was chiefly 



