394 



HUTCHINSON'S POPULAR BOTANY 



yellow and dark purple colours for dung- and carrion-flies." A more 

 familiar example is our own little Cuckoo-pint (Arum maculatuni). All 

 such flowers give out an offensive odour which serves as an additional 

 attraction to carrion-loving flies. 



Physiologically speaking, the cause of the colours of flowers (as, indeed, 

 of fruits also) is the existence of pigments in their cells, either dissolved 



or in solid particles. The colouring 



principles are 'the same as in autumn 

 leaves namely, anthocyanin and 

 phylloxanthin. Thus the yellow hue 

 of the florets of the Dandelion is due 

 to microscopic bodies of protoplasm 

 tinged with a yellow pigment, which 

 are contained in the epidermal cells ; 

 while the colour of the Pheasant' s-eye 

 (Adonis autumnalis) is due to a red 

 pigment. In the last-named flower 

 the particles are less than FTT Vo^ f 

 an inch in diameter. In blue, violet, 

 and (so-called) black flowers (which 

 are probably in most cases a deep 

 shade of violet), the colouring matter 

 is usually dissolved in the cell-sap. 

 White flowers do not derive their 

 whiteness from any pigment, but from 

 the fact that their epidermal cells are 

 filled with air. Professor Thome has 

 shown in the case of Zinnia elegans 

 how peculiarly the separate pigments 

 of a flower may be distributed in 

 different superimposed layers of cells. 

 " The marginal florets of the capitulum 

 are scarlet on the upper and light 

 yellow on the under side, while the 

 layers of cells nearest the surface 

 contain a purple sap with orange 

 granules. The rest of the cells are 



filled with a colourless sap in which float a smaller number of light 

 yellow granules." 



The changes in the colours of flowers, like the changes in the colours 

 of leaves, are due to oxidation. The Common Borage (Borago ojficinalis} 

 changes from pink to blue ; and the pale pink bells of the young flowers 

 of the Garden Convolvulus (Ipomcea ptirpurea) frequently change to a 

 deep purple as they mature. These are familiar instances of oxidation 



FIG. 484. SAXIFRAGE (Saxifraga 



officinalis). 



The white corolla has two violet spots at the bast * 



each petal. Pollinated by short-lipped insects, 



especially flies. 



