THE METABOLISM OF PLANTS. 241 



Sachsse is of opinion that the starch is converted into etiolin, 

 that the starch undergoes oxidative decomposition, that fatty 

 aldehydes and aromatic substances (e.g. pyrocatechin) are 

 among the products, and that these combine to form etiolin ; 

 under the influence of light etiolin is reduced to chlorophyll. 



It is not probable, however, that this is an accurate 

 account of the chemical changes in process. It has been 

 shewn (p. 154) that chlorophyll contains not only C, H, and 

 O, but also N, and probably also P, in its molecule ; and 

 reference has been made to Hoppe-Seyler's suggestion that 

 chlorophyll is a body allied to the lecithins. It is true that 

 we have no trustworthy analysis of etiolin, but all that is 

 known on the subject tends to shew that etiolin and chloro- 

 phyll are closely-allied bodies: what is here said about 

 chlorophyll may therefore be regarded as true of etiolin also. 

 In consideration of the complex composition of these colour- 

 ing-matters, it is probably nearer the truth to regard them 

 as derivatives, not of the starch, but of the protoplasm of the 

 corpuscle. From this point of view the correlated disappear- 

 ance of the starch and the formation of the colouring-matter 

 is to be explained thus : that as protoplasm is consumed 

 in the formation of the colouring-matter, the starch is used 

 in the construction of fresh protoplasm. With regard to the 

 nature of the change which etiolin undergoes on its conver- 

 sion into chlorophyll, we have no definite information. (The 

 spectrum of etiolin is given in the plate.) 



It has been ascertained by Pringsheim, Wiesner, and 

 others, that green parts of plants always contain both etiolin 

 and chlorophyll. Besides these, a third substance, xantho- 

 phyll, is commonly present. Pringsheim has shewn that 

 xanthophyll is probably a derivative of chlorophyll, though 

 the exact relation between the two is not determined. It 

 is to xanthophyll that the autumnal colouration of leaves is 

 principally due : in many cases the leaves also contain a red 

 colouring-matter, erythrophyll, dissolved in the cell-sap. 



The colouring-matters of flowers and of fruits are sometimes confined 

 to protoplasmic corpuscles, and sometimes they are dissolved in the 

 cell-sap : the former is the case with the yellow, orange, and brown 



V. 16 



