9 8 



THE COLOURS OF FLOWERS. 



when flowers revert to wind-fertilisation, both disuse 

 and natural selection cause them to lose their petals, 

 and become simply green. 



In practice, however, it is often hard to distinguish 

 between the casually entomophilous, the self-fertilised, 

 and the really anemophilous species ; and they are 

 so intermixed that it may perhaps be best to consider 

 them together. For example, the common ash (Fraxi- 

 nus excelsior, Fig. 31) belongs to a gamopetalous 

 family, the Oleacece, and is closely related to the white 

 privet (Ligustrum vulgare), which has conspicuous 



FIG. 31. 



-Hermaphrodite, male and female flowers of English ash (Fraxinus 

 excelsior); purplish no petals. 



white flowers. But many large trees, owing, perhaps, 

 to their long life, and consequent less necessity for 

 producing many seeds, tend to lose their petals ; and 

 this is remarkably the case among the olive group. 

 The shrubby species have usually flowers with a four- 

 lobed corolla ; and so have many of the southern ar- 

 boreal forms (Fig. 32) : but the northern trees, like our 

 ash, have lost both calyx and corolla altogether, each 

 naked flower consisting only of two stamens, with a 

 single ovary between them. In appearance their 

 blossoms seem of much the same sort as the wind- 



