REPRODUCTION AND DISPERSAL 899 



cinths. Coloration takes place in the dark in some non-bulbous plants, 

 such as Lychnis, Hydrangea, and Papaver. In striking contrast to 

 bulbous plants are Antirrhinum and Prunella, where the food necessary 

 for anthesis does not accumulate during the previous season, but is 

 manufactured just before the period of floral development. In such 

 plants the flowers do not become fully colored when the entire plant is 

 grown in the dark, as in the tulip, although they become colored if the 

 vegetative shoots are grown in the light and the floral shoots in the dark. 

 Even tulip flowers do not become colored in the dark unless the previous 

 leaf generation is exposed to sunlight. Thus the influence of light upon 

 color intensity appears to be in part direct, as is indicated by the deep 

 shades of hyacinth flowers that are grown in the sunlight and by the 

 high intensity of color of alpine flowers. However, to an equal or greater 

 extent the light influence is indirect, as is well shown by those flowers 

 that become colored only when the leaves are in the light. The color 

 in this case and in the dark cultures of bulbous plants seems to be asso- 

 ciated with a rich food supply, a fact which is quite in harmony with the 

 sugar theory of anthocyan formation. Probably in the majority of flow- 

 ers, direct exposure to sunlight is required to produce full coloration, 

 although a certain amount of pigmentation occurs in darkness. Yellow 

 colors are much less weakened by darkness than are the anthocyan 

 colors. Heat as well as light affects coloration, the intensity often being 

 heightened at low temperatures. 



Variations in the quality or kind of color are much less understood 

 than are variations in color intensity, though it is known that the cell 

 sap of red anthocyan flowers is more acid than is the cell sap of blue 

 anthocyan flowers; hence it is to be supposed that factors which cause 

 variations in the acidity of the cell sap cause variations in color also. 

 The flowers of Hydrangea hortensis, which usually are red, become blue 

 when the plants are grown in soil containing a considerable amount of 

 the sulfates or of other salts of aluminum and potassium. Aluminum 

 salts frequently change lilac-colored flowers to blue, whereas potassium 

 salts may change them to green. Acids, on the other hand, frequently 

 change flower colors to red. White roses have been changed to red by 

 adding potassium salts to the soil. Heat affects the quality as well as 

 the intensity of flower color, low temperatures, for example, sometimes 

 causing white geraniums to become red or rose geraniums to become 

 carmine. The flowers of harebells and morning-glories also vary with 

 the temperature in respect to color quality (see also p. 845). 



