XI 



THE SPECIAL COLOURS OF PLANTS 



311 



Fig. 28. 



difterent plants, forming the class Dioecia of Linnaeus. In these 

 cases the pollen may be carried to the stigmas either by the 

 ^\dnd or by the agency of insects. 



Now these four methods arc all apjxxrently very sim})le, 

 and easily produced by varia- 

 tion and selection. They are 

 applicable to flowers of any 

 shape, recjuiring only such size 

 and coloiu' as to attract insects, 

 and some secretion of nectar 

 to ensure their repeated visits, 

 characters common to the great 

 majority of flowers. All these 

 methods are common, except 

 perhaps the second ; but there 

 are many flowers in which the 



nnllpn frmn qnnfhpr nl-inf i^; ^falva sylvestris, Malva rotumlifolja, 

 poiien II om anoinei piant Ib adapted for insect- adapted for self- 



prepotent over the pollen from fertUisation. leitilisation. 



the same flower, and this has nearly the same effect as self- 

 sterility if the flowers are frequently crossed by insects. We 

 cannot help asking, therefore, why have other and much 

 more elaborate methods been needed? And how have the 

 more complex arrangements of so many flowers been brought 

 about ? Before attempting to answer these questions, and in 

 order that the reader may appi'eciate the difiiculty of the 

 l^roblem and the nature of the facts to be explained, it will be 

 necessary to give a summary of the more elaborate modes of 

 seciu-ing cross-fertilisation. 



(1) We first have dimorphism and heteromorphism, the 

 phenomena of which have been already sketched in our 

 seventh chapter. 



Here we have both a mechanical and a physiological 

 modification, the stamens and pistil being variously modified 

 in length and position, while the diff'erent stamens in the same 

 flower have widely different degrees of fertility when applied 

 to the same stigma, — a phenomenon which, if it were not so 

 well established, would have appeared in the highest degree 

 improbable. The most remarkable case is that of the three 

 different forms of the loosestrife (Lythrura salicaria) here 

 figured (Fig. 29 on next page). 



