446 History of the Theory of [Book hi. 



of growth. But this gave no insight into the nature of the 

 substances from which plants prepare their food. On this 

 point Mariotte endeavoured to give such information as could 

 be obtained from the chemistry of his day ; and he has the 

 merit of having shown, in opposition to the old Aristotelian 

 notion, that plants convert the food-material which they derive 

 from the ground into new chemical combinations, while the 

 earth and the water supply the same elements of nutrition to 

 the most different kinds of plants. It could not escape the 

 notice of physiologists even of that time, that the water which 

 plants take up from the ground introduces into them but very 

 small quantities of matter in solution. Van Helmont in the 

 first half of the 17th century had shown this by an experiment, 

 the results of which, however, led him to think that plants 

 were able to produce both the combustible and incombustible 

 parts of their substance from water. Hales at the beginning 

 of the 1 8th century formed a different opinion, being led by 

 the evolution of the gases in the dry distillation of plants to 

 conclude, that a considerable part of their substance was 

 absorbed in a gaseous form from the atmosphere. 



The views propounded by Malpighi, Mariotte, and Hales 

 contained the most important elements of a theory of the 

 nutrition of plants ; fully understood they would have taught 

 that one part of the food of plants comes from the earth and 

 the water, and another part from the air ; that the leaves 

 change the materials thus obtained in such a manner as to 

 produce from them the substance of plants and to apply this 

 to the purposes of growth ; but the ideas were not combined 

 in this way, for during some years after their time botanists 

 were chiefly engaged in observations on the movement of the 

 sap in plants, and they arrived even on this point at very 

 obscure and even contradictory results, because they overlooked 

 the function of the leaves which had already been recognised 

 by Malpighi. All insight not only into the chemical processes 

 in the nutrition of plants, but also into the mechanical laws of 



