90 A TEXTBOOK OF BOTANY [Ch. Ill, 11 



of two chemical substances, carotin and xanthophyll 

 proper, though sometimes additional yellow pigments are 

 present. Carotin and xanthophyll have the property of 

 relatively high stability in light, on which account they 

 show forth in full intensity when the more unstable chloro- 

 phyll, which is made only while the leaf is in full health, 

 fades away in the light. 



The white colors of leaves represent simply the natural 

 color of composition of the leaf structure when all colored 

 pigments are absent. The white is translucent in cells which 

 contain sap, but is silvery in those which are dead and 

 filled with air, as in some variegated Begonias. White 

 areas cannot, of course, form food, and are rare in wild 

 plants; but they have been greatly intensified in cultiva- 

 tion, in the striped and variegated foliage of Begonias, fancy- 

 leaved Caladiums, and Ribbon Grasses. Sometimes the 

 same leaves contain also areas or stripes of red, thus increas- 

 ing the variegation, as occurs very prominently in the re- 

 cently-developed Rainbow Corn. 



Various colors appear also in leaves as result of the action 

 of parasites, either Fungi or Insects. In some cases the color 

 belongs to the parasite itself, as in the Rust of Wheat leaves, 

 where it resides in the rusty-red spore masses. More com- 

 monly it results from damage done to the complicated metab- 

 olism of the leaf by the parasite, followed by disappearance 

 of chlorophyll, and consequent exposure of the yellow 

 xanthophyll; or the tissues may be killed altogether, and 

 hence soon display their distinctive decay color, which is 

 brown. Colors due to injury by parasites may usually 

 be recognized by a certain abnormal or unhealthy aspect 

 they give to the leaf, and especially by their wholly irregular 

 or asymmetrical distribution in relation to the leaf structure. 



Most striking and interesting, however, of all the non- 

 green leaf colors is the autumnal coloration of foliage, which 

 constitutes one of the major phenomena of nature. Its 

 foundation lies in the fact that with waning vitality, brought 



