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310 DARWINISM chap. 



"Nature abhors perpetual self -fertilisation.^ In this principle 

 we appear to have a sufficient reason for the various con- 

 trivances by which so many flowers secure cross-fertilisation, 

 either constantly or occasionally. These contrivances are so 

 numerous, so varied, and often so highly complex and extra- 

 ordinary, that they have formed the subject of many elaborate 

 treatises, and have also been amply pojDularised in lectures 

 and handbooks. It Avill be unnecessary, therefore, to give 

 details here, but the main facts will be summarised in order 

 to call attention to some difficulties of the theory Avhich seem 

 to require further elucidation. 



Modes of securing Cross-Fertilisation. 



When we examine the various modes in which the cross- 

 fertilisation of flowers is brought about, Ave find that some are 

 comj^aratively simple in their operation and needful adjust- 

 ments, others highly com^jlex. The simple methods belong to 

 four principal classes : — (1) By dichogamy — that is, by the 

 anthers and the stigma becoming matm'e or in a fit state for 

 fertilisation at slightly difterent times on the same plant. The 

 result of this is that, as plants in different stations, on diff'erent 

 soils, or exposed to difterent aspects flower earlier or later, the 

 mature pollen of one plant can only fertilise some plant 

 exposed to somewhat diff'erent conditions or of different con- 

 stitution, whose stigma "svill be mature at the same time ; and 

 this difference has been shown by Darwin to be that which is 

 adapted to secure the fullest benefit of cross -fertilisation. 

 This occurs in Geranium pratense. Thymus serpyllum, Arum 

 maculatum, and many others. (2) By the flower being 

 self-sterile "with its own pollen, as in the crimson flax. This 

 absolutely prevents self-fertilisation. (3) By the stamens and 

 •-' ' anthers being so placed that the pollen cannot fall upon the 

 stigma, while it does fall upon a visiting insect which carries 

 it to the stigma of another flower. This effect is produced in 

 a variety of very simple ways, and is often aided by the 

 motion of the stamens which bend down out of the way of 

 the stigmas before the pollen is ripe, as in INIalva sylvestris 

 Lf-- (see Fig. 28). (4) By the male and female flowers being on 



^ For the full detail of liis experiments, see Cross- and Self-Fertilisation 

 of Plants, 1876. 



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