482 Theory of the Nutrition [Bookiii. 



Hales is the last of the great naturalists who laid the 

 foundations of vegetable physiology. Strange as some of their 

 ideas may seem to us, yet these observers were the first who 

 gained any deep insight into the hidden machinery of vegetable 

 life, and handed down to us a knowledge both of individual facts 

 and of their most important relations. If we compare what 

 was known before Malpighi's time with the contents of Hales' 

 book, we shall be astonished at the rapid advance made in less 

 than sixty years, while scarcely anything had been contributed 

 to the subject in the period between Aristotle and Malpighi. 



3. Fruitless attempts to explain the movement of the 

 sap in plants. 1730-1780. 



If those, who studied the nutrition of plants and especially 

 the movement of their sap in the period between Hales and 

 Ingen-Houss, had kept a firm hold on Malpighi's view, that 

 the nutritive substances are elaborated in the leaves, and had 

 combined it with Hales' idea, that plants derive a large portion 

 of their substance from the air, they would have had a principle 

 to guide them in their investigations into the movement of the 

 sap ; and by experimenting on living plants they might have 

 succeeded in giving a more definite expression to these ideas, 

 even though chemistry and physics supplied during that time 

 no new aids. We have said already that such was not the 

 course of events ; physiologists confined their attention to the 

 obvious phenomena of vegetation, and trusted in so doing to 

 gain a firmer footing, but in this they never got beyond a 

 commonplace and unreflecting empiricism, because their 

 observation was without an object, and their conclusions 

 without a principle. They wandered from the right direction, 

 as always happens when observation is not guided by a well- 

 considered hypothesis ; and their conceptions were rendered 

 more obscure by their imperfect acquaintance with one of the 

 most important aids to understanding the movement of the 



