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Show R72 MEANS Ol!' CROSS-FERTILISATION. CHAP. X. of soeds by any means, and the second, cross fertilisation. The ad vantages derived fro In cross-fertilisation throw a flood of light on most of the c~ief chara?ters of flO\:ers. We can thus understand theu large size and bnght colours, and in soino cases tho bright tints of tho adjoining parts, such as the peduncles, ~ractem, &_c. By this means they are rendered conspicuous to Insects, on tho saine principle that almost every fruit .which is devoured by birds presents a strong contrast 1n colour with the green foliage, in order that it may be seen, and its seeds freely disseminated. With some flowers conspicuousness is gained at the expense even of the reproductive organs, as with the ray-florets of many Oompositm, the exterior flowers of Hydrangea, and th~ terminal flowers of the Feather-hyacinth or Muscan. There is also reason to belieYe, and this was the opinion of Sprengel, that flowers differ in colour in accordance with the kinds of insects which frequent thorn. Not ou.l y do the bright colours of flowers serve to attract insects but dark-coloured streaks and marks are often pres;nt, which Sprengellong ago maintained served as guides to the nectary. Theso marks follow tho voins in the petals~ or lie between them. They Ina y occur on only one~ or on all excepting one or 1nore of the upper or lower petals; or they n1ay form a dark ring round the tubular part of the corolla, or be confined to the lips of an irr gular flower. ~n. th~ white varietie& of n1any flowers, such as of Dttgtt~al'ts p~~rpurea, Antirrhinun~ ma.fus, several species of ~Ianthus, Phlox, Myosotis, Rhododendron, Pelargoni~m, Pri1nula and Petunia., the marks generally persist, whilst the rest of tho corolla has become of a pure whito ; but this may Le due merely to thei~ coloudr being more intense and thus less rea dI.l Y ob l It erate · CHAP. X. MEANS OF CROSS-FERTILISATION. 37:J Sprengel's notion of the use of these marks as guid 8 appeared to me for a long time fanciful; for insect.·, without such aid, readily discover and bite holes through the · nectary from the outside. Thoy also discover the Ininute nectar-secreting glands on the stipules and leaves of certain plants. Moreover, son1o few plants, such as certain poppies, which are not nectariferous, havo guiding marks; but we might perhaps expect that some few plants would retajn traces of a former nectariferous condition. On the other hand, these marks are much more common on asymn1ctrical flowers, the entrance into which would be apt to puzzle insects, than on regular flowers. Sir J. Lubbock has also proved that bees readily distinguish colours, and that they lose much time if the position of honey which they have once visited be in tho least changed.* The following case affords, I think, the best evidence that these marks have really been developed in correlation with the nectary. 1'he two upper petals of the common Pelargonium are thus marked near their bases; and I have repeatedly observed that when the flowers vary so as to become peloric or regular, they lose their nectaries and at the same time the dark marks. When the nectary is only partially aborted, only one of the upper petals loses its mark. Therefore the nectary and these marks clearly stand in some sort of close relation to one another ; and the simplest view is thnt they were developed together for a special purpose ; the only conceivable one being that the 1narks serve as a guide to the nectary. It is, however, evident fr01n what has been already said, that insects could discover the nectar without the aid of guiding marks. They are of service to the plant, only by aiding insects to * ' Bl'itish vVilcl Flowers in relation to Ins2cts,' 1875, p. 4-1.. |
