CHAP, ii Photosynthesis 301 



that it disproved the view that the carbon dioxide can 

 be absorbed either altogether or in part from the soil by 

 the roots. 



Sachs summed up his views in the following sentence : 



' According to my theory, the basis for the whole of the 

 organic substance of a normal plant is provided by the 

 formation of starch in the chlorophyll of the organs of 

 assimilation, and above all the whole of the carbon, in 

 whatever organic combination it may be found later, occurs 

 originally in the form of starch.' (Vorlesungen, Eng. ed., 

 p. 312.) 



It is not to be wondered at that the promulgation of 

 this view, which, though not now held, marked a very great 

 advance in the direction of the truth, led many observers 

 to speculate upon the range of chemical change and the 

 probable sequence of events in the passage from the simple 

 substances water and carbon dioxide to the very complex 

 carbohydrate starch, a compound so complex that its con- 

 stitution was not at the time grasped by chemists them- 

 selves. Theories of the stages of construction, at first 

 purely hypothetical, were slowly formed. There was but 

 little to go upon ; the substances from which the construc- 

 tion was effected had been determined, but no information 

 further was available except that the appropriation of the 

 carbon of the carbon dioxide is attended by the exhalation 

 of a volume of oxygen approximately equal to that of the 

 carbon dioxide concerned. 



In 1861 Berthelot and Kekule suggested that the first 

 stage must consist of the formation of formic acid or some 

 other member of the formyl group, but the credit of devising 

 the first complete hypothesis, covering the whole process, 

 was due to Baeyer in 1870. This remarkable hypothesis 

 was in many respects supported by experiment, and 

 was still the most prominent one at the close of the 

 century. It was the following: the carbon dioxide and 



