THE PRODUCTS OF THE ASSIMILATION OF CARBON DIOXIDE 317 



Other pigments. Molisch has recently isolated the phycoerithrin of red 

 seaweeds and the phycocyanin of the Cyanophyceae, and has shown them to be 

 crystallizable proteids. It is possible that the phycophaein of brown seaweeds 

 and other chloroplastic pigments which are soluble in water may also be proteid 

 in nature 1 . 



SECTION 54. The Products of the Assimilation of Carbon Dioxide. 



It is at present impossible to say what the first-formed product in 

 the assimilation of carbon dioxide may be ; moreover, it may never 

 accumulate to any extent, but may immediately undergo further modifica- 

 tion in various directions, although the first stage of carbon dioxide assimi- 

 lation is probably identical in all green plants. It is impossible, however, to 

 say whether the same path is followed in all cases until sugar is produced, 

 or whether similarity is exhibited only as far as the production of some 

 simple body, from which, according to circumstances, sugar, oil, starch, 

 proteids, or other visible products may be formed. Many facts indicate 

 that a general agreement is shown in the early production of a carbo- 

 hydrate, but nevertheless various assimilatory products may appear in 

 a cell, owing to the more or less marked modifications which this carbo- 

 hydrate, probably sugar, may subsequently undergo. 



Physiology can only deal with the visible products of assimilation, and 

 these have precisely the same nutritive value and metabolic importance as 

 similar food supplies obtained from the external world. It is, however, an 

 extremely difficult task to discover what is the precise manner in which this 

 nutriment is produced by the functional activity of the chloroplastid, and it 

 seems improbable that proteid synthesis should be necessarily connected with 

 the assimilation of carbonic acid. On the other hand, carbohydrates may be 

 formed by the decomposition of proteids, and hence it is not impossible that 

 the sugar which appears in the chloroplastid may be derived from proteids 

 of antecedent origin. Similarly, oil may be formed from starch, as well as 

 starch from oil, but it is not certain whether these and other processes can 

 be carried on by the chloroplastid itself without the aid of the external 

 cytoplasm. Thus, if the chloroplastid were unable to form starch from 

 anything but sugar, then starch could only be formed from glycerine if the 

 cytoplasm previously converted this substance into sugar. In any case the 

 insoluble starch must necessarily be formed at the expense of some soluble 

 product, and since many chlorophyllous and non-chlorophyllous plastids can 

 form starch grains when supplied with sugar (Sects. 53) 55), it is obvious that 

 starch can hardly be the primary product of carbon dioxide assimilation. 



1 Molisch, Bot. Zeitung, 1894, p. 181. See also Zimmermann, Beihefte z. Bot. Centralbl., 1894, 

 Bd. IV, p. 91; Molisch, Bot. Zeitung, 1895, p. 131; Hansen, ibid., 1884, p. 649; Reinke, ibid., 

 1886, p. 177 ; Schiitt. ibid., 1887, p. 259. 



