46 HISTORY OF BOTANY 



circulation of sap, and efforts were constantly being made 

 to draw a comparison between plant and animal in this 

 relation. Ray, indeed, sought, but sought in vain, for 

 valves that might regulate and direct the flow, and denied 

 their existence only after he found he could induce water 

 to pass down through the wood, as well as upwards and 

 laterally. 



Another physiological investigator of some note was 

 Christian Wolff, who, in the early years of the eighteenth 

 century, followed out some of the problems touched 

 upon by Mariotte, Grew, and Malpighi. He studied the 

 relation of the plant to the soil and observed that the 

 latter was impoverished by repeated cultivation of crops 

 upon it without periodic manuring. Rain, he thought, 

 carried down substances into the soil, and the materials 

 so added were absorbed by the root. While holding, 

 with Mariotte, that capillarity was the chief cause of 

 ascent of sap in the vessels of the wood he adds expansion 

 of air as another factor in the process. 



While Wolff may be regarded rather as a critic than 

 as a discoverer, we must place a contemporary of his, 

 Stephen Hales, in the very " front rank as an experimenter. 

 I have already pointed out to you how Grew, while 

 laying the foundations of vegetable anatomy, had con- 

 stantly in mind the solution of physiological problems, 

 a task in which he cannot be said to have been very 

 successful. Hales, on the other hand, tackled physio- 

 logical questions, using as the foundation of his work 

 Grew's anatomical results ; he is, in short, Grew's counter- 

 part from the point of view of function, and the works 

 of these two men taken together may be regarded as re- 

 presenting the state of plant anatomy and physiology in 

 the first two or three decades of the eighteenth century. 



Hales was born in the year in which Grew's Anatomy 

 was published, viz., 1671, and died ninety years after- 

 wards. For over forty years he lived the retired life 

 of a country vicar, devoting all his leisure time to research 



