THE SPECIAL COLOURS OF PLAXTS 



311 



by the 

 simple, 



Fig, 

 Malva sylvestris, 

 adapted for insect- 

 fertilisation. 



28. 



Malva rotnndifolia, 

 adapted for self- 

 fertilisation. 



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

 cases the pollen may be carried to the stigmas either 

 mnd or by the agency of insects. 



Now these four methods are all apparently very 

 and easily produced by varia- 

 tion and selection. They are 

 applicable to flowers of any 

 shape, requiring only such size 

 and colour 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 

 pollen from another j^lant is 

 prepotent over the pollen from 

 the same flower, and this has nearly the same efl'ect 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 appreciate the difficulty of the 

 problem and the nature of the facts to be explained, it will be 

 necessary to give a summary of the more elaborate modes of 

 secui'ing cross-fertilisation. 



(1) AVe 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 jDistil being variously modified 

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

 flower have widely diff'erent degrees of fertility when applied 

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

 well estaldished, would have appeared in the highest degree 

 improbable. The most remarkable case is that of the three 

 different forms of the loosestrife (Lythrum salicaria) here 

 figured (Fig. 29 on next page). 



