462 



ANIMAL BIOLOGY 



The less obvious structural and functional differentiation of 

 plants retarded progress in plant physiology as it did in plant 

 anatomy. Probably of most historical, and certainly of most gen- 

 eral interest is the development of our knowledge of the nutrition 

 of green plants. Aristotle's notion that the plant's food is pre- 

 pared for it in the ground was still prevalent during the seven- 

 teenth century when Malpighi, from his studies on plant histology, 

 gave the first hint of supreme importance — the crude ' sap ' en- 

 ters by the roots and is carried to the leaves where, by the action 



Fig. 301. — Stephen Hales. 



of sunlight, evaporation, and some sort of a 'fermentation,' it is 

 elaborated and distributed as food to the plant as a whole. 



It is Hales (1667-1761) of England, however, to whom the 

 botanist looks as the Harvey of plant physiology, because in his 

 Vegetable Staticks (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 considerable part of their food from the at- 

 mosphere, and also that the leaves play an active role in the move- 

 ments of fluids up the stem and in eliminating superfluous water 

 by evaporation. Still the picture was incomplete, and so remained 

 until the biologist had recourse to further data from the chemist. 

 (Fig. 301.) 



