GENERAL 309 



established the essential features of the process of carbon dioxide assimilation, and 

 broadened and deepened our knowledge of this phenomenon. De Saussure proved 

 decisively that the assimilation of carbon dioxide provides the green plant with 

 organic food, and that the power of respiration which all living plants exhibit is 

 a totally different phenomenon. Respiration and carbon dioxide assimilation were 

 not, however, always kept distinct, although Dutrochet and Meyen regarded them 

 as being separate processes quite independent of one another (see Sects. 50, 95). 

 The ' humus theory ' in vogue at the time prevented de Saussure from recognizing 

 the universal importance of photosynthetic assimilation, and it was not until this 

 theory had been upset that the importance of carbon dioxide assimilation in the 

 economy of nature was fully recognized (see Sect. 95). 



When Senebier, de Saussure, and Ingenhousz had clearly established the fact 

 that all green parts can decompose carbonic acid gas, whereas all uncoloured parts 

 and the coloured non-chlorophyllous petals of flowers, <Scc. have not this power, 

 it became possible to establish the dependence of carbon dioxide assimilation on 

 the presence of chlorophyll, and to explain the possession of this power by red 

 foliage leaves l as being due to the presence of chlorophyll in them. De Saussure 

 probably held this opinion, although his excessive caution prevented him from 

 definitely expressing it. Dutrochet ", however, regarded the power of carbon 

 dioxide assimilation as being directly dependent upon the presence of chlorophyll, 

 and from this time onwards the same teaching has been repeated in all tlie better 

 text-books. Cloez 3 showed subsequently that coloured leaves are only able to 

 assimilate carbon dioxide by means of the chlorophyll which they contain. 



Senebier, de Saussure, and de Candolle discussed more or less fully the 

 further changes which the organic food thus obtained must undergo in metabolism 4 , 

 and as soon as it was certain that all organic food was obtained in this manner, 

 it became evident that the assimilatory products must undergo most intricate and 

 complicated metamorphoses, in order to produce the different complex substances 

 which the plant contains. Various hypotheses have been put forward concerning 

 the primary products of carbon dioxide assimilation (Pringsheim, 1. c., p. 67). 

 Mohl, Unger, and Boussingault concluded that these were carbohydrates, but it 

 was Sachs who first showed that the starch which appears in chloroplastids exposed 

 to light is formed by the assimilation of carbon dioxide 5 . The importance 

 of Sachs' researches is in no wise affected by the fact that starch is not always 



1 Shown first by Senebier and Saussure. Further researches by Coremvinder, Compt. rend., 

 1863, T. LVII, p. 268. 



2 Dutrochet, Memoires, &c., Bruxelles, 1837, p. 186. Thus Mohl, 1851 ; Unger, 1855, &c. Cf. 

 Pringsheim, 1. c., pp. 26, 45, &c. 



3 Cloez, Compt. rend., 1863, T. LVII, p. 834; Ann. d. sci. nat., 1863, iv. ser., T. xx, p. 184. 



4 Senebier, Physiol. veget., 1800, T. iv, p. 165; de Candolle, Physiol. (Roper\ 1833, T. I, 

 pp. 117, 139, 170, &c. 



5 Mohl, Grundziige d. Anat. u. Physiol., 1851, p. 45. Unger (ibid., 1855, P- 265) gives 

 chemical equations to explain the production of carbohydrates and free oxygen. Sachs, Bot. Zeitung, 

 1862, p. 368 ; 1864, p. 288. Mohl (Vermischte Schriften, 1845, P- 355 > Bot. Zeitung, 1855, p. 115) 

 and Nageli (Die Starkekorner, 1858, p. 398) do not bring the appearance of starch into genetic 

 relationship with the assimilation of carbonic acid. 



