EPOCHS IN BIOLOGICAL HISTORY 397 



of the nutrition of green plants. Aristotle's notion that 

 the plant's food is prepared for it in the ground was still 

 prevalent during the seventeenth century when Malpighi, 

 from his studies on plant histology, gave the first hint of 

 supreme importance the crude sap enters by the roots 

 and is carried to the leaves where, by the action of sunlight, 

 evaporation, and some sort of a fermentation, it is elaborated 

 and distributed as food to the plant as a whole. 



Fio. 204. Stephen Hales. 



It is STEPHEN HALES (1677-1761) of England, however, to 

 whom the botanist looks as the Harvey of plant physiology, 

 because in his Vegetable Statics (1727) he laid the foundations 

 of the physiology of plants by making " plants speak for 

 themselves" through his incisive experiments. For the 

 first time it became clear that green plants derive a con- 

 siderable part of their food from the atmosphere, and also 

 that the leaves play an active role in the movements of 

 fluids up the stem and in eliminating superfluous water by 

 evaporation. Still the picture was incomplete, and so it 



