298 The Physiology of Plants BOOK in 



that the chlorophyll mechanism is something independent 

 of the general body of both animal and plant. This is 

 supported further by Ewart's observations in 1896 of the 

 conditions under which inhibition can be brought about. 

 Whenever such inhibition was caused by changes in the 

 environment, or by the administration of drugs, the failure 

 could be traced directly to the plastid, the cell in which 

 it was found not being affected, or not to the same extent. 



The mechanism seems therefore to have been shown to 

 be an independent apparatus. It is, however, equally 

 certain that it is highly specialized ; so much so indeed 

 that its death or destruction slowly or rapidly follows 

 a disturbance of the relations of acclimatization between 

 the plastid and the cell. 



Other pigments have been supposed to be concerned 

 under certain conditions in the liberation of oxygen. In 

 some cases they have been associated with the ordinary 

 plastid and appear to have taken the place of chlorophyll 

 in it. Draper showed in 1878 that etiolated plants evolve 

 oxygen, and in 1881 Engelmann traced the power to the 

 etiolated plastids. The observation was confirmed in 1897 

 by Ewart. In both cases the method used was Engel- 

 mann's bacterium method. The power according to both 

 observers is very feeble. The observation gives some sup- 

 port to the view already stated, that etiolin is an ante- 

 cedent of chlorophyll in the plastid. Like the latter it 

 is formed under the influence of light. Elfving's observa- 

 tion on the result of illumination at a low temperature has 

 already been alluded to. 



Kohl, in 1897, observed that both xanthophyll and carotin 

 exhibit a similar power. 



A pigment not associated with a plastid was said by 

 Engelmann in 1888 to be able to cause the exhalation of 

 oxygen. This was a colouring matter found in certain 

 purple bacteria and named by its discoverer bacterio-pur- 



