THEORETICAL 355 



of light, as is indicated by the fact that starch may be formed from glucose 

 in darkness, but those changes which lead to an evolution of oxygen seem 

 to be directly dependent upon the action of light-energy, and the continued 

 removal of the gas produced allows the process to continue uninterruptedly, 

 just as an energetic decomposition of silver iodide is induced by light only 

 when the iodine is removed as fast as it is liberated : . 



Hitherto attention has almost solely been paid to the immediately 

 perceptible green dye chlorophyll, which though an important is still 

 simply a co-operative member in the assimilatory mechanism (Sect. 52). 

 The view held by certain authors that chlorophyll is the primary assimi- 

 latory product is no longer tenable (Sect. 54), and Pringsheim's protective 

 theory has also proved to be erroneous (Sect. 59). Nor has Wiesner's 

 supposition been confirmed, according to which it is the chlorophyll which 

 directly liberates oxygen from carbon dioxide, while since pure chlorophyll 

 contains no iron, Horsford's suggestion that iron may exercise the necessary 

 reducing action can no longer be admitted 2 . 



Various theories have been put forward which are fairly plausible 

 from a chemical point of view, but decisive evidence can only be obtained 

 by direct observations upon the living organism, and at present these 

 do not suffice to allow any positive conclusions to be drawn. A certain 

 amount of probability attaches to Bayer's theory, according to which 

 formic aldehyde is produced in the chloroplastid from carbon dioxide and 

 water, oxygen being evolved, while from this formic aldehyde (CH 2 O) 

 carbohydrates may be formed by polymerization 3 . The latter process 

 is one which may be comparatively readily induced, but no evidence 

 has as yet been brought forward to show that formic aldehyde is 

 actually produced by the assimilation of carbonic acid. Even if plants are 

 able to form starch when supplied with this substance it does not follow 

 that it is the primary assimilatory product (Sect. 55), for various sub- 

 stances may induce a deposition of starch in chloroplastids. Moreover 



1 Cf. Ostwald, I.e., p. 1080. 



2 Wiesner, Sitznngsb. d.Wien. Akad., 1874, Bd. LXIX, Abth. i, p. 59 of Sep.-abdr. Cf. Pfeffer, 

 Osmot. Unters., 1877, p. 166. Other hypotheses are given by Kraus (Flora, 1875, p. 269) and by 

 Timiarezeff (Bot. Zeitung, 1885, p. 619); Hereford, Sitzungsb. d. Wien. Akad., 1873, Bd. LXXVIJ, 

 Abth. ii, p. 436. See also Ballo, Ber. d. Chem. Ges., 1889, p 750. The fantastic hypotheses of 

 Kisler (Jahresb. d. Chem., i8-;9. p. 560) and of Benkovich (Ann. d. Phys. u. Chem., 1875, Bd.CLlv, 

 p. 468) hardly require discussion. 



3 Bayer, Ber. d. Chem. Ges., 1870, Bd. Ill, p. 66. Cf. Meyer und Jacobson, Lehrb. d. organ. 

 Chemie, 1893, Bd. l, p. 401 ; also ref. in Bot. Zeitung, 1886, p. 849. The simple proof of the presence 

 of aldehydes in the plants affords no positive evidence. See also E. Fischer, Ber. d. Chem. Ges., 

 1894, p. 3231. Bach (Compt. rend., 1893, T. CXVI, pp. 1145, 1389) states that formal aldehyde 

 may be produced in dead masses by the action of sunlight, but these results are in urgent need of 

 confirmation. [Curtius and Reinke (Ber. d. Chem. Ges., 1897, p. 201) find traces of aldehydes 

 to be present in all chlorophyllous parts exposed to light, but none in Fungi, and none in seedlings 

 grown in darkness, whether etiolated or green, although these substances soon appear in the seed- 

 lings when exposed to light.] 



A a 2 



