n6 THE BIOLOGY OF THE SEASONS 



tion, one of the most familiar being that some flowers, such 

 as Grass of Parnassus, give forth incense only in the sunshine, 

 while others, like the Evening Campion, reserve this for the 

 night. 



After discussing the volatile ethereal oils to which the 

 odours of plants are due, Professor S. H. Vines says : " With 

 regard to the function and fate of these aromatic substances, 

 it appears that they are of no use in the constructive pro- 

 cesses ; they are to be regarded as waste-products, destined, 

 for the most part, to be thrown off. . . . We may venture 

 upon the general statement that the higher plants, at least, 

 cannot avail themselves of carbon when combined in an 

 aromatic molecule for the purposes of their constructive 

 metabolism. . . . Although the aromatic substances are 

 probably to be regarded simply as waste-products, yet some 

 of them are indirectly of use to the plant. We have seen 

 that the odours of plants are due to the presence of volatile 

 ethereal oils, and it has been ascertained that the odours of 

 flowers serve to attract insects, and thus contribute to ensure 

 fertilisation." l 



The third accessory characteristic of the flower is its 

 colour, which appears to be attractive to many of the insect- 

 visitors. Let us consider the colour, first of all, in its internal 

 or physiological aspects. It is due to a great variety of 

 pigments, some of which are more intelligible than others. 

 Some are fixed in protoplasmic corpuscles, notably those of a 

 yellow, orange, brown (and rarely blue) colour. These are 

 mostly derivatives of the leaf-green or chlorophyll, and the 

 yellow anthoxanthin is a common example. The others are 

 dissolved in the cell-sap, notably those of a white, violet, 

 blue, red (and rarely yellow) colour. Most of these are deriva- 

 tives of tannin and other bitter principles, and the blue 

 anthocyanin is a common example. 



i S, H, Vines, Lectures on the Physiology of Plants, Cambridge, 1886. 



