[Chap. IV SEASONAL ASPECTS OF PLANTS 31 



50^ F. may fail to form chlorophyll, and unless the temperature is raised 

 they ultimately die. 



Environmental effects on carotinoids. The formation of the yellow pig- 

 ments is also dependent upon certain environmental conditions. The 

 seedlings of some varieties of com and other plants are conspicuously 

 yellow when growing in darkness; others, such as oats, are colorless 

 under the same conditions and become yellow only when exposed to 

 light. Perhaps there are certain factors that might be substituted for light 

 in these cases, but they have not been discovered. Seedlings in which 

 chlorophyll is made in the dark also contain carotinoids. 



Anthocyanins and the environment. The roots of some plants, such as 

 beets and radish, become red or purple in darkness; but light is neces- 

 sary for the formation of anthocyanins in most leaves and fruits. In 

 some instances the blue and violet rays are necessary. In both leaves 

 and fruits intensity of color is increased by abundance of light, relatively 

 low temperature, and a low supply of nitrates in the soil. Apparently 

 these environmental conditions influence the formation of anthocyanins 

 partly through their influence on the sugar content of the cells. A high 

 sugar content seems to be one of the conditions within the cells neces- 

 sary to the formation of anthocyanins. 



Leaves and fruits on the same tree or on the same variety of tree in 

 different local situations may differ widely in the amount of anthocyanin 

 they contain. Peaches in the top of the tree, and apples fully exposed to 

 the sunlight are redder than those inside the crown and shaded by the 

 outer foliage. The more intense coloring of fruits from the Northwestern 

 States as compared with those from the Eastern States exemplifies this 

 same principle. Low night temperatures are also very important in the 

 accumulation of sugar and the formation of bright colors in apples. 



Low temperatures increase anthocyanin formation in many exergreen 

 plants; for example, the leaves of certain varieties of juniper and arbor 

 vitae become copper-colored in autumn, and similar changes occur in 

 many heaths, of which the cranberry is an example (Plate III). 



The intensity of anthocyanin colors of flowers may vary with light 

 intensity. The red color in some flowers is associated with an acid 

 condition, and blue with an alkaline condition. If a red geranium petal 

 is crushed on a blotter and held alternately near ammonia and acetic 

 acid, one may see these color changes. Similar color changes may also 

 be seen in the uncrushed petals. This change in color does not occur in 

 all anthocyanins because of the presence of certain ions, such as potas- 



