PLANT MANUFACTURE OF FOOD 137 



acid and an alcohol, containing carbon, hydrogen, oxygen, 

 nitrogen and magnesium. There is a striking similarity between 

 chlorophyll and the red pigment hemoglobin which is found in 

 blood. Both decompose to form the same products, and the chief 

 chemical difference between the two is that chlorophyll contains 

 magnesium and hemoglobin contains iron. Two other pigments 

 — carotin and xanthophyll — are associated with chlorophyll. 

 Carotin, reddish yellow in color, is abundantly produced in 

 carrots, a few fruits and most orange colored flowers; chemically 

 it is a hydrogen and carbon compound. Xanthophyll, made up 

 of hydrogen, carbon and oxygen, is more yellow than carotin. 

 Both of these pigments frequently occur in special colored 

 plastids, the chromoplasts. The oranges and golden-yellows of 

 autumn foliage are due to the exposure of the carotin and 

 xanthophyll which are not as easily decomposed by the low tem- 

 peratures as the chlorophyll, which disappears as the leaf 

 vitality wanes. If no accessory pigments are present in leaves — 

 as in the case of birches, ashes, sycamores and poplars — trees 

 usually assume these yellow hues in autumn. The brown pigment 

 FucoxANTHiN (cf. p. 88) characteristic of the Brown Algae, is 

 chemically related to carotin. 



Some plant pigments are combinations of sugars with other 

 substances, and are known as glucosides. Such are the soluble 

 anthocyan pigments found in cell sap, which are responsible 

 for the red and blue colors found in plants. Anthocyans are 

 abundant in the roots of beets and the outer tissues of many 

 fruits. They depend for their formation upon the presence of 

 sugar and of light, especially the ultra-violet end of the spectrum. 

 Therefore fruits depend upon sunlight for development of red 

 colors; it is a well-known fact that apples and peaches will not 

 turn red if kept in the dark. The value of the anthocyans to the 

 plant is not exactly known. When they occur in flowers they 

 obviously are of value in attracting pollinating insects. In some 

 cases the pigment may aid in screening delicate tissues from too 

 intense sunlight; anthocyans are abundant in young foliage 

 shoots of some trees, such as the niaples. In still other cases, the 

 pigment absorbs the sun's rays and transforms them into heat, 

 thus protecting the plant against low temperatures. Arctic plants 



