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 18th 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
