XI 
THE SPECIAL COLOURS OF PLANTS 
311 
different plants, forming the class Dicecia of Linnaeus. In these 
cases the pollen may he carried to the stigmas either by the 
wind or by the agency of insects. 
Now these four methods are all apparently very simple, 
and easily produced by varia¬ 
tion and selection. They are 
applicable to Rowers 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 fiow r ers in which the 
l'lollpn from nnntbpr ntmt L Malva sylvestris, Malva rotundifolia, 
poneil II om anomei plant, IS adapted for insect- adapted tor self- 
prepotent over the pollen from fertilisation. fertilisation. 
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 1 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 
securing cross-ferti 1 isation. 
(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 different 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 (Lythrum salicaria) here 
figured (Fig. 29 on next page). 
