REPRODUCTION AND DISPERSAL 845 



large quantities of water, which accumulates in the outer floral organs, 

 giving rise to the term water calyx. 



In a few cases insects visit flowers for other kinds of food than pollen or nectar, 

 as in certain orchids (e.g. Maxillaria), where there occur on the lip of the corolla 

 fragrant hairs rich in fatty and albuminous foods. Some flowers and inflorescences 

 develop a considerable degree of heat at anthesis, and it has been claimed that cer- 

 tain insects visit them for nocturnal shelter and warmth. Since most of the con- 

 spicuous heat-producing flowers and inflorescences are found among tropical palms 

 and aroids, this view seems untenable. 



Floral features accessory to pollination. — Color. — The role of the 

 pistil and the stamens is very obvious: the protective and synthetic role 

 of the calyx also is obvious (p. 869), but the role of the corolla is far less 

 evident. The corollas of flowers, taken as a whole, are ephemeral 

 organs whose evanescence is due to their extreme delicacy and conse- 

 quent easy wilting, and to their early abscission, much after the manner 

 of deciduous foliage leaves. Corollas present a most bewildering luxu- 

 riance of form, color, and marking without parallel elsewhere among 

 plant organs. Most colors except black and green occur commonly, 

 and flowers therefore contrast sharply with the foliage. Reds and blues 

 are due to anthocyans dissolved in the cell sap, the former indicating 

 maximum acidity and the latter minimum acidity; indeed, certain flowers, 

 as in Lychnis, vary in color with the varying acidity of the cell sap. 

 Some yellow flowers owe their color to pigments related to the antho- 

 cyans and like them dissolved in the cell sap. Orange colors and many 

 yellow colors are due to plastids colored with carotin, xanthophyll, 

 or with related pigments (fig. 755). Brown colors are due commonly 

 to a combination of plastid and sap pigments. Flower pigments are 

 believed to be oxidation products, and whiteness, which denotes the 

 absence of pigment, arises where the necessary oxidizing ferment (oxi- 

 dase) is absent, or, if present, is neutralized by reducing agents. The 

 peculiar color-like effect of white flowers is due to the presence of air in 

 the petals or to unequal reflection and refraction. Nocturnal flowers 

 especially are likely to be white, and many species, whose flowers com- 

 monly vary from blue to red, may produce white sports, known as albinos. 

 The more or less fundamental distinction between the anthocyan (or 

 cyanic) flowers and the yellow (or xanthic) flowers is shown by the fact 

 that species and even genera rarely change from one to the other; for 

 example, hepaticas and asters, with all their variations, are not yellow, 

 or goldenrods and sunflowers cyanic. The cyanic colors would seem to 



