COLOURS OF PLANTS. 233 



glacialis; the saxifrages of the high. Alps with bunches of 

 flowers a foot long as in Saxifraga longifolia and S. coty- 

 ledon, or forming spreading masses of flowers as in S. oppo- 

 sitifolia; while the soapworts, silenes, and louseworts 

 are equally superior to the allied species of the plains. 



Why Allied Species of Flowers Differ in Size and 

 Beauty. Again, Dr. Miiller has discovered that when 

 there are showy and inconspicuous species in the same 

 genus of plants, there is often a corresponding difference 

 of structure, those with large and showy flowers being 

 quite incapable of self-fertilization, and thus depending 

 for their very existence on the visits of insects ; while 

 the others are able to fertilize themselves should insects 

 fail to visit them. We have examples of this difference 

 in Malva sylvestris, Epilobium augustifolium, Poly- 

 gonum bistorta, and Geranium pratense which have 

 all large or showy flowers, and must be fertilized by 

 insects as compared with Malva rotundifolia, Epilo- 

 bium parviflorum, Polygonum aviculare, and Geranium 

 pusillum, which have small or inconspicuous flowers, 

 and are so constructed that if insects should not visit 

 them they are able to fertilize themselves ?* 



Absence of Colour in Wind-fertilized Flowers. As 

 supplementing these curious facts showing the relation 

 of colour in flowers to the need of the visits of insects 

 to fertilize them, we have the remarkable, and on any 

 other theory, utterly inexplicable circumstance, that in 

 all the numerous cases in which plants are fertilized 

 by the agency of the wind they never have specially 

 coloured floral envelopes. Such are our pines, oaks, 

 poplars, willows, beeches, and hazel ; our nettles, grasses, 



1 Nature, vol. ix. p. 164. 



