328 PIGMENTS 



olive colour of the magnesium-free compounds, phaeophytin 

 a and b (for formulae, see p. 319). Boil the solution with a 

 very small crystal of copper acetate and note the bright green 

 colour is restored when the magnesium is replaced by copper. 



THE CAROTINOIDS OR YELLOW PIGMENTS 

 ACCOMPANYING CHLOROPHYLL. 



The term carotinoid or lipochrome is applied to a group of 

 yellow orange or brown pigments which are widely distri- 

 buted in the plant and animal worlds. In the plant they 

 occur either associated with chlorophyll in the chloroplasts or 

 else in plastids by themselves ; they are to be found in all 

 types of plant both phanerogams and cryptogams, and in the 

 latter group, particularly in those members which are without 

 chlorophyll, such as the fungi, they are frequently responsible 

 for the colour. In the animal world they are responsible for 

 the yellow colour of fats, hence the term lipochrome ; the 

 yellow pigment of the corpus luteum of the cow is identical 

 with carotin, whilst the yellow pigment of egg yolk and of 

 blood serum of fowls is known to be xanthophyll. The follow- 

 ing members of this group have been described : carotin, 

 lycopin (a red isomer of carotin), xanthophyll, which probably 

 exists in four modifications known as a, a', a" and jS which 

 differ slightly in their absorption spectra, rhodoxanthin (a red 

 isomer of xanthophyll), and fucoxanthin. 



CAROTIN, C40H56. 



This pigment is widely distributed and, as has already been 

 mentioned, is generally associated with chlorophyll in the 

 chloroplasts. It also occurs in various forms, amorphous or 

 crystalline, in various parts of many plants. The colour of 

 yellow or orange petals is not infrequently due to it, e.g. the 

 corona of the common Narcissus, A'^. Poeticus ; similarly the 

 presence of innumerable small intracellular crystals of carotin 

 are responsible for most of the colour of the root of the carrot, 

 and so also is the tint of many fruits where the carotin is often 

 in amorphous granules. 



With regard to the physiological significance of carotin 



