(10/10. Quoted in 'Gardeners' Chronicle' 1846 page 771.) With Mirabilis, three or four of the very large pollen-grains are sufficient to fertilise an ovule; but I do not know how many grains a flower produces. With Hibiscus, Kolreuter found that sixty grains were necessary to fertilise all the ovules of a flower, and he calculated that 4863 grains were produced by a single flower, or eighty-one times too many. With Geum urbanum, however, according to Gartner, the pollen is only ten times too much. (10/11. Kolreuter 'Vorlaufige Nachricht' 1761 page 9. Gartner 'Beitrage zur Kenntniss' etc. page 346.) As we thus see that the open state of all ordinary flowers, and the consequent loss of much pollen, necessitate the development of so prodigious an excess of this precious substance, why, it may be asked, are flowers always left open? As many plants exist throughout the vegetable kingdom which bear cleistogene flowers, there can hardly be a doubt that all open flowers might easily have been converted into closed ones. The graduated steps by which this process could have been effected may be seen at the present time in Lathyrus nissolia, Biophytum sensitivum, and several other plants. The answer to the above question obviously is, that with permanently closed flowers there could be no cross-fertilisation.
The frequency, almost regularity, with which pollen is transported by insects from flower to flower, often from a considerable distance, well deserves attention. (10/12. An experiment made by Kolreuter 'Forsetsung' etc. 1763 page 69, affords good evidence on this head. Hibiscus vesicarius is strongly dichogamous, its pollen being shed before the stigmas are mature. Kolreuter marked 310 flowers, and put pollen from other flowers on their stigmas every day, so that they were thoroughly fertilised; and he left the same number of other flowers to the agency of insects. Afterwards he counted the seeds of both lots: the flowers which he had fertilised with such astonishing care produced 11,237 seeds, whilst those left to the insects produced 10,886; that is, a less number by only 351; and this small inferiority is fully accounted for by the insects not having worked during some days, when the weather was cold with continued rain.) This is best shown by the impossibility in many cases of raising two varieties of the same species pure, if they grow at all near together; but to this subject I shall presently return; also by the many cases of hybrids which have appeared spontaneously both in gardens and a state of nature. With respect to the distance from which pollen is often brought, no one who has had any experience would expect to obtain pure cabbage-seed, for instance, if a plant of another variety grew within two or three hundred yards. An accurate observer, the late Mr. Masters of Canterbury, assured me that he once had his whole stock of seeds "seriously affected with purple bastards," by some plants of purple kale which flowered in a cottager's garden at the distance of half a mile; no other plant of this variety growing any nearer. (10/13. Mr. W.C. Marshall caught no less than seven specimens of a moth (Cucullia umbratica) with the pollinia of the butterfly-orchis (Habenaria chlorantha) sticking to their eyes, and, therefore, in the proper position for fertilising the flowers of this species, on an island in Derwentwater, at the distance of half a mile from any place where this plant grew: 'Nature' 1872 page 393.) But the most striking case which has been recorded is that by M. Godron, who shows by the nature of the hybrids produced that Primula grandiflora must have been crossed with pollen brought by bees from P. officinalis, growing at the distance of above two kilometres, or of about one English mile and a quarter. (10/14. 'Revue des Sc. Nat.' 1875 page 331.)
All those who have long attended to hybridisation, insist in the strongest terms on the liability of castrated flowers to be fertilised by pollen brought from distant plants of the same species. (10/15. See, for instance, the remarks by Herbert 'Amaryllidaceae' 1837 page 349. Also Gartner's strong expressions on this subject in his 'Bastarderzeugung' 1849 page 670 and 'Kenntniss der Befruchtung' 1844 pages 510, 573.