372 MEANS OF CROSS-FEKTILISATION. CHAP. X. 



in a quite subordinate degree, is the wind ; and with 

 some aquatic plants, according to Delpino,* currents of 

 water. The simple fact of the necessity in many cases 

 of extraneous aid for the transport of the pollen, and the 

 many contrivances for this purpose, render it highly 

 probable that some great benefit is thus gained ; and 

 this conclusion has now been firmly established by the 

 proved superiority in growth, vigour, and fertility of 

 plants of crossed parentage over those of self-fertilised 

 parentage. But we should always keep in mind that 

 two somewhat opposed ends have to be gained; the 

 first and more important one being the production 

 of seeds by any means, and the second, cross-ferti- 

 lisation. 



The advantages derived from cross-fertilisation throw 

 a flood of light on most of the chief characters of flowers. 

 We can thus understand their large size and bright 

 colours, and in some cases the bright tints of the 

 adjoining parts, such as the peduncles, bracteae, even 

 true leaves, as with Poinsettia, &c. By this means 

 they are rendered conspicuous to insects, on the same 

 principle that almost every fruit which is devoured by 

 birds presents a strong contrast in colour with the 

 green foliage, in order that it may be seen and its 

 seeds freely disseminated. With some flowers con- 

 spicuousness is gained at the expense even of the 

 reproductive organs, as with the ray-florets of many 

 Composite, the exterior flowers of Hydrangea, and the 

 terminal flowers of the Feather-hyacinth or Muscari. 

 There is also reason to believe, and this was the opinion 

 of Sprengel, that flowers differ in colour in accordance 

 with the kinds of insects which frequent them. 



* See also Dr. Ascherson's interesting essay in ' B( t. Zeitung,' 

 1871, p. 444. 



