CHAPTER VI 



THE DEVELOPMENT OF KNOWLEDGE OF THE 

 METABOLIC PROCESSES 



WE have seen that as late as 1860 hardly any accurate 

 knowledge of the constructive processes had been obtained. 

 For several years it had been known that the leaves are 

 the chief seat of formation of what were generally but 

 obscurely called elaborated substances, and it had been 

 inferred that for the growth of the general plant body the 

 formative matter must be conducted thence. The idea of 

 a general correspondence of the streams of material to and 

 from the leaves with the circulation of the blood of the 

 higher animals had seized the imagination of many, and 

 had led to the use of the phrase ' the circulation of the 

 sap '. In some writings of the time we find the view put 

 forward that the raw sap, or as they called it the ' crude ' 

 sap, ascends through the cells and vessels of the stem and 

 root, undergoes an elaboration in the leaves under the 

 influence of light and air, and then descends through the 

 bark into the roots again, whence, after losing somewhat 

 by excretion, the rest mixes with the entering stream. 

 The descending stream or ' Elaborated sap ' was held by 

 Hartig and by de Candolle, as late as 1858, to be a kind 

 of mucilage or gum, from which by some process of con- 

 centration or evaporation of large quantities of water, solid 

 reserve materials were deposited in various places in the 

 plant. 



The problem facing investigation in this direction in 

 1860 was stated by Sachs as being mainly twofold the 

 nature of the substances produced in the leaves, and the 



