Physiology of Plants. 71 



of water into roots may be compared with its rising in 

 capillary tubes; that the endosperm in the seed may be 

 likened to the yolk in an egg; and that the prevalent 

 conception of a vegetable soul was a gratuitous hypo- 

 thesis. 



A few experiments by John Ray showing the upward 

 passage of sap in the wood and its lateral movement as 

 well; Woodward's measurements showing how much 

 water a mint may take up by its roots and discharge by 

 evaporation; Christian Wolff's acute observations on the 

 exhaustion of the soil after much has been grown on 

 it, and on the variety of matters contained in rain-water 

 are all of interest, but they are "thrown into the 

 shade by the brilliant investigations of Stephen Hales 

 (1677-1761), in whom we see once more the genius of 

 discovery and the sound original reasoning powers of 

 the great explorers of nature in Newton's age" (Sachs). 

 His Vegetable Statics (1727) may be called the founda- 

 tion-stone of plant physiology. 



Hales deserves a most honourable place in the history 

 of physiological botany, not merely because he was a 

 pioneer at an early date, but because he indicated the 

 only sure path of progress. He brought rigorous physi- 

 cal methods to bear upon a biological problem. By 

 ingenious experiments and careful measurements he 

 " made his plants themselves speak". His investiga- 

 tions on the ascent of sap remain of interest, and he was 

 the first to prove that a great part of the food of plants 

 must be derived from the air. It must be remembered, 

 of course, that physics and chemistry had made some 

 progress, else Hales could not have secured his foothold. 



In spite of the admirable beginnings made by Mal- 

 pighi, Mariotte, and Hales, vegetable physiology degen- 

 erated for nearly half a century into profitless theorizing 

 about circulation and the like. A new impulse was 

 needed, and that came from chemistry, which Lavoisier 

 had begun to reorganize. In 1774 Priestley (1733-1804) 

 had discovered oxygen, and five years later he showed 

 that this gas was, in certain conditions, exhaled by 

 plants. In the same year Ingen-Houss (1730-1799) took 

 an even bigger stride, showing that it is only in the light 



