PHYSIOLOGICAL 411 



(B) Chemical or pigmentary coloration, due to the presence 

 of a substance whose molecules absorb or reflect only a certain part 

 of the visible spectrum, may be illustrated by the green colour 

 of most plants, the reddish pigment of our blood, the blackish 

 pigment of dark fur, and the yellowish pigment of yolk of egg. 



(C) But the finest coloration, in animals at least, is due to 

 a combination of physical and pigmentary colour, as in the blue 

 feathers of some parrots, the metallic green wing-cases of some 

 beetles, the beautiful blue scales of some butterflies, and the 

 iridescent bristles of Aphrodite, the sea-mouse. When physical 

 structure is a factor accessory to pigmentation, the colour of the 

 surface changes as it is moved about, as is familiar in peacock's 

 feathers : when physical structure enhances the effect of a pigment, 

 it may result in a colour, notably blue or green, which is quite 

 different from the colour of the pigment. With this introduction 

 let us now pass to consider pigments in their chemical aspect. 



Chemistry of Organic Pigments. — ^This study is relatively 

 modern and it is still in the early stages of its development. Ex- 

 cellent pioneering work was done many years ago in Krukenberg's 

 Vergleichend-Physiologische Studien and Vortrdge (1881-1889), but 

 his followers, such as Sir F. Gowland Hopkins, have not been many. 

 An admirable introduction to the subject is furnished by Dr. Marion 

 Newbigin's Colour in Nature (London, 1898). 



Chlorophyll Pigments. — It is convenient to begin with chloro- 

 phyll, the green colouring matter of plants, for it is the most 

 important pigment in the world, being vitally connected with 

 photosynthesis. The molecule of chlorophyll is still very imperfectly 

 understood, but it is readily split into two parts by the action of 

 an alkali. One of the parts is a complex, colourless alcohol called 

 phytol. The other part has for its foundation the p>Trol ring 

 (C-C-C-C-N), in which four carbon atoms form a ring with one atom 

 of nitrogen. In chlorophyll there are four of these pyrrol rings 

 joined together, and linked to these in some way there is a single 

 atom of magnesium. The reason for giving this detail is that a 

 very similar structure recurs in the red pigment of the blood! 

 As a matter of fact the chlorophyll of plants is much more compli- 

 cated than used to be supposed; it is indeed a system of four pig- 

 ments. Two of these, chlorophyll-rt and chlorophyll- &, absorb the 

 red and orange rays and therefore appear green. Besides these 

 there are two yellowish pigments, carotin and xanthophyll, which 

 seem to be less important. When the light shines on the green leaf, 

 there is probably a continuous cycle of change, from chlorophyll-^ 

 to chlorophyll-6, with absorption of carbon dioxide, and then back 

 again from chlorophyll-^ to chlorophyll-a, with liberation of oxygen. 

 This is of great interest, because in the blood of Vertebrates the 

 red pigment shows the same general alternation — between oxy- 



