70 The Science of Life. 



and yet the "humus-theory", which persisted into the 

 nineteenth century, was as grossly erroneous. It may 

 be noted, however, that what we now know in regard 

 to the role of bacteria (not to speak of earth-worms) 

 in preparing the soil-food for plants might be used to 

 rehabilitate both the Aristotelian conjecture and the 

 humus-theory. 



Towards the end of the sixteenth century (1583) Ces- 

 alpino broke away from the bondage of Aristotelian 

 tradition. He compared the vessels and fibres of plants 

 to the veins in animals, and suggested that the food 

 passed into and through the plant by a sort of suction, 

 as oil in the wick of a lamp. Joachim Jung also marks 

 the growing revolt. He insisted that the plant took an 

 active part in its own nourishment, and suggested that 

 the nature of the openings in the root might be such 

 as to admit only what was of advantage. The chemist 

 Van Helmont (1577-1644) deserves to be remembered in 

 this connection as the author of the first recorded experi- 

 ment in vegetable physiology. He planted a willow in 

 a weighed quantity of soil and watered it with rain; in 

 five years the plant had grown from 5 Ibs. in weight to 

 164, while the earth in the pot showed only a loss of 

 2 ounces. Not suspecting that the plant drew a great 

 part of its food from the air, he was forced to exaggerate 

 the virtues of rain-water. 



J. D. Major (1639-1693) is generally referred to as the 

 founder of the theory of circulation in plants a subject 

 of discussion all through the eighteenth century, and by 

 no means beyond dispute still; but we reach firmer 

 ground in the work of the keen-sighted histologist 

 Malpighi. To him is due the first suggestion of the 

 fundamental fact that the leaves elaborate the crude sap; 

 he believed that this passed from the roots to the leaves 

 by the fibrous elements of the wood ; and his only gross 

 error was in regarding the wood-vessels as respiratory 

 air-tubes. 



Equally important were the conclusions of the physi- 

 cist Mariotte (d. 1684), who maintained, for instance, 

 that different plants draw the same material from the 

 soil, but make different stuffs out of it; that the entrance 



