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Show 310 DARWINISM CHAJ'. "Nature abhors perpetual self-fertilisation.! In this yrinciplc we appear to have a sufficient reason for the va,n?':s ~ontrivances by which so many flowers secure cro~. -fert1hsatwn, either constantly or occasionally. These contnvances arc so numerous, so varied, and often so highly complex and oxtra,ordinary, that they have formed the subject of. man:y elaborate treatises and have also been amply populansed m lectur ·s and ha~dbooks. It will be unnecessary, therefore, to give details here but the main facts will be summarised in orcl e1· to call atte~tion to some difficulties of the theory which cern to require further elucidation. Modes of securing C1·oss-FeTtilisation. When we examine the various modes in which the cro. sfertilisation of flowers is brought about, we find that some a,rc comparatively simple in their operat~on and needful adjmstments others highly complex. The simple methods belong to four ~rincipal classes :-(1) B1 dichogamy--:-that is, by the anthers and the stigma becommg mature or m a fit state for fertilisation at slightly different times on the same plant. The result of this is that, as plants in different stations, on different soils or exposed to different aspects flower earlier or later, the mat~re pollen of one plant can only fertilise some plant exposed to somewhat different conditions or of diffe:·ent <..'<>11- stitution, whose stigma will be mature at the same t1me; an<l 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 fla,x. Thi · absolutely prevents self-fertilisation. · (3) By the stamens and anthers being so placed that the pollen cannot faJl upon the stioma while it does fall upon a visiting insect which earries it ~o the stigma of another flower. This effect is produced in a variety of very simple ways, and is often aided hy the motion of the stamens which bend down out of the way of the stigmas before the pollen is ripe, as in Malva syl.vcstris (see Fig. 28). ( 4) By the male and female flowers bcmg 0 11 I For the full detail of his experiments, see Cross- and • 'elf-Fertilisation of Plants, 1876. X[ THE SPECIAL COLOURS OF PLANTS 311 different plants, forming the class Dicecia of Linnaeus. In these ca.scs the pollen may be carried to the stigmas either by the wmd or by the agency of insects. Now. these four methods are all apparently very simple, and eas1ly produced by variation and selection. They arc 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 F1r.. 2 ' . Pollen frOffi another ])lant iS ~( al\·a sylvcstri s, ~J a lv::t I"O(llll<lifoJia, ada pted for i11scct- nda ptNL for ~ e lf- prepotent over the po1len from fertilisation. fertilisation. the same flower, and this h<ts nearly the same effect <ts selfsterility if the flowers are frequently crossed by insects. We cannot help a king, 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 securing cross-fertilisation. (1) We first h:.~.ve dimorphism and heteromorphi m, the phenomena of which have been already sketched in our seventh chapter. ~ere . we have both a mechanical and a physiological ~od1ficatwn, the stamens and pi til being variously modified m 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 ~ell 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). # |
