the growth of the roots, buds, and fruit, must be conducted to those parts from the leaves. It could no longer be a question whether such a movement of assimilated material takes place; it remained only to consider what are the conducting tissues, and what is the nature of the substances which are produced in the leaves and conducted to the rest of the organs. Both questions in accordance with the organisation of the plant could be properly answered only by microchemical methods, and these were not adopted and further developed till after 1857. We have already said that nothing certain was known even as late as 1860 about the chemical combinations formed by assimilation in the leaves; De Candolle supposed that the primary formative sap was a gum-like substance, from which the rest of the various vegetable substances were secreted in the different tissues. Theodor Hartig, who had done good service between 1850 and 1860 by his investigations into the starch in the wood of trees and into proteid in seeds, by the discovery of sieve-tubes, by observations on the amount of water in woods at different times of the year, and by other contributions to botanical science, also occupied himself with the subject of the descending sap, which he conceived of as a formless primary mucilage, from which, as from De Candolle's gum, the various substances in the plant were deposited as it travelled through the plant. He says ('Botanische Zeitung' for 1858, p. 341), 'The crude sap is changed in the leaves into primitive formative sap,' and 'the formation of solid reserve-material (from this) involves the elimination of large quantities of watery fluid.' The occasional remarks of vegetable physiologists of all sorts between 1840 and 1860 prove, that similar ideas respecting the formation of a primary mucilage of this kind in the leaves were generally entertained.
