48 CAROTINOIDS AND RELATED PIGMENTS 



the laboratory, the constant presence of these pigments in the chloro- 

 plastid is very difficult to explain unless the constitution of one type 

 of pigment bears a simple relation to that of the other. The various 

 theories which have been offered regarding the possible functions of 

 the carotinoids in the chloroplastid also fall down unless the caro- 

 tinoids are closely relationcd chemically. Ewart (1915), to be sure, 

 has recently claimed to have succeeded in reducing xanthophyll to 

 carotin in the laboratory. The evidence for this is very unconvincing, 

 especially in view of the fact that Ewart on subsequent study (1918) 

 failed to substantiate any of the other products which he first claimed 

 to have produced from xanthophyll on photo-oxidation. The reduc- 

 tion experiment of xanthophyll to carotin unfortunately was not 

 repeated in the second study. 



Carotinoids in Etiolated Leaves 



The yellow chroniolipoids which develop without chlorophyll in the 

 leucoplastids, when plaiiN are grown in the dark, would seem to be 

 closely related to. if not completely identical with those found in the 

 chloroplastids, at lea>t qualitatively, inasmuch as etiolated plants form 

 chlorophyll very rapidly in the li<rht without loss of yellow constitu- 

 ents in the re.-ultini chloroplastids. No studies have been made of the 

 pigments of etiolated leaves, however, since our newer chemical con- 

 ceptions of the plant carotinoids have arisen so that it is necessary 

 to depend upon older investigations for our experimental knowledge 

 of these coloring matter^. It is possible to state with certainty that 

 carotin is present in the etiolated plant, but the evidence is insuffi- 

 cient to substantiate the belief of Tammes (1900) and Kohl (1902f) 

 that it is probably the only yellow chromolipoid present inasmuch as 

 it is now known that the methods which these investigators employed 

 are not specific for carotin. 



Joannes Rajus (1693) appears to have first recorded the observation 

 that plants which grow in the dark do not turn green but have a 

 yellow color. Bonnet (1754) named such plants "plantes etiolces." 

 The question whether the yellow pigment or some colorless substance 

 was the forerunner of the green pigment which developed so rapidly 

 when etiolated plants are exposed to the light occupied the attention 

 of many investigators. Interest in this question was stimulated by the 

 discovery of Phipson (1858) that etiolated leaves rapidly assume an 

 emerald green color when immersed in con. H 2 S0 4 . Phipson followed 



