Theories of Photo-synthesis. 207 



separate out had a greenish colour owing to its physical condition. 

 When dissolved, it shows no trace of chlorophyll. It was further 

 found that a liquid rich in xanthophyll could contain traces of 

 chlorophyll in finely suspended form without any traces of chloro- 

 phyll being visible under the spectroscope. This is partly because 

 owing to the opacity of a liquid containing waxy impurities in 

 finely suspended form, only thin layers can be examined. When 

 such liquids are evaporated, and the green solids which first sepa- 

 rate out removed and dissolved, the appearance is given of a pro- 

 duction of chlorophyll having taken place. In other observations 

 in which chlorophyll appeared to be regenerated with the agency of 

 zinc dust, the apparent regeneration was due to the formation of the 

 ^inc compound of phaeophytin, which closely resembles chloro- 

 phyll. 



Hitherto the regeneration of chlorophyll from its magnesium 

 containing derivatives, and from xanthophyll or carotin under the 

 action of light, has not been possible outside the living plant. 

 Iwanowski^ states that solutions of chlorophyll containing carotin 

 or xanthophyll are more stable towards light than chlorophyll alone. 

 This may be partly due to a direct protective action, carotin in 

 particular combining more readily with oxygen than chlorophyll 

 ■does. According to Iwanowski, however, the protective action is 

 more marked when both carotin and xanthophyll are present. This 

 may possibly be because a certain amount of regeneration of chloro- 

 phyll takes place with the aid of light energy from the products 

 of its decomposition and from those of the photo. -oxidation of caro- 

 tin and xanthophyll. 



In the case of those plants which can develop chlorophyll in dark- 

 ness, its building up requires a supply of energy which is either 

 derived from the oxidation of the stored food materials, or which 

 is absorbed directly as heat from outside. It is worthy of note that 

 in all cases the minimum temperature for the formation of chloro- 

 phyll is higher than for the formation of carotin or xanthophyll, 

 and plants which can turn green in darkness will do so at a lower 

 temperature in light than they wnll in darkness. 



Until the heats of combustion of chlorophyll, glaucophyllin. 

 phyllophyllin, phytyl, carotin, and xanthophyll are known, it is 

 impossible to discuss the energy changes which may be involved in 

 carbon dioxide assimilation. A large part at least of the energy 

 represented by the carbohydrates produced is undoubtedly light 

 energy, which was absorbed and used in the reconstruction of chloro- 



1 Ber. d. D. Bot. Ges., 1913-14, 31, pp. 600-613. 



