THE STRUCTURE AND FUNCTION OF HIGHER PLANTS 77 



Simple experiments show that the color of these substances depends partly 

 upon the amount of acid present. If the center of a head of red cabbage, 

 which is rich in these anthocyanin pigments, is cooked, and to the liquid 

 obtained a little vinegar is added(vinegar is acetic acid), the juice of the 

 cabbage will become bright red; if a little ammonia is poured in, the solu- 

 tion becomes blue to yellowish green. The greater the acidity, the deeper 

 the red color will be. 



Various factors are responsible for the development of these red and 

 blue pigments. First there is the genetic makeup of the plant. Maples in- 

 herit the ability to manufacture these substances, while hickories do not; 

 the petals of the buttercup are never red, while those of the scarlet sage 

 rival the faces of the most highly embarrassed. 



For the most part, light is important in the formation of the anthocy- 

 anins. The sunny side of an apple is brighter than the shady side; the "stem 

 end" is more richly colored than the "flower end." Perhaps in days gone 

 by you pasted your initials in opaque paper on green apples, and when the 

 fruits ripened there were the letters in green on a background of red. The 

 more modern version of this, as described by Arthur, is to paste Cellophane 

 on a green apple, put India ink marks on the cellophane, and then expose 

 the apple to a suitable lamp; the skin of the apple shaded by the ink re- 

 mains green. Photographic negatives have been printed on the skins of 

 apples in shades of red and green, using sunlight as a light source. 



It has long been known that red leaves of the Virginia creeper contain 

 more sugars than green leaves on the same plant collected at the same 

 time. In 1 899 Overton put the leaves or leafy stems of various plants, such 

 as some species of lily, of holly and of columbine, into sugar solutions, 

 and after some weeks they became red. Injured branches of trees often 

 become bright red, while the rest of the tree is still green, presumably 

 because the sugars manufactured by the leaves are not transported away, 

 and consequently stay where they are made. Abundance of sugars favors 

 the development of the red pigments. 



It is common knowledge that brisk weather — without prolonged frost 

 — is conducive to rich coloration, and experimental work supports this 

 general observation. Both with leaves and flowers this is true. A blue bell- 

 flower and a red primrose were pale, almost white, when grown under 

 warm greenhouse conditions. Cool weather, though preferably not too 

 frosty, stimulates colors in autumn leaves as well as in healthy youngsters. 



Though abundant water favors the growth of plants, it does not result 

 in brilliant coloration. Of course drought is fatal, but a degree of dryness 

 toward the end of the season results in beautiful colors. Experimentally 

 this can be shown by watering sparingly some plants that are "vulnerable," 

 and by watering others lavishly; the former will have the red pigments 

 more strikingly developed. 



Although leaves rich in sugars are often briUiantly colored, plants grow- 



