238 



Composites Dahlia. 



offers one of the most striking instances of the variability o.? 

 species under domestication, which is exemplified not only in 

 the modification of the disk-florets but also in the wide range of 

 colours. But so little is understood of the real nature of vege- 

 table colouring matter and the cause of its variability, and to 

 what influences the changes must be ascribed, that we cannot 



Fig. 127. Dahlia coccinea. (-J- nat. size.) 



correctly estimate the importance of this phenomenon. We 

 know that pure white flowers exist, and that various shades and 

 tints of yellow, scarlet, and purple, and combinations of these 

 colours, are common ; but we are not sure whether these colours 

 are not also found in natural varieties. There is evidently a 

 limit in the production of colours, as nothing approaching blue 



