534 Theory of the Nutrition of Plants. 



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 i860 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 i860 by his investigations into the 

 starch in the wood of trees and into proteid in seeds, by the dis- 

 covery of sieve-tubes, by observations on the amount of water 

 in woods at different times of the year, and by other contribu- 

 tions 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 physio- 

 logists of all sorts between 1840 and i860 prove, that similar 

 ideas respecting the formation of a primary mucilage of this 

 kind in the leaves were generally entertained. 



