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Show 376 MEANS OF CROSS-FERTILISATION. CUAP. X. phorus-the two most precious of all the ele1nents for the growth of plants-but in the case of most open flowers, a large 'quantity of pollen is consumed by pollen-devouring insects, and a large quantity is destroyed during long-continued rain. With many plants this latter evil is guarded against, as far as is possible, by the anthers opening only during dry weather,*-by the position and form of some or all of the petals,-by the presence of hairs, &c., and as Korner has shown in his interesting essay,t by the n1ovmnents of the petals or of the whole flower during cold and wet weather. In order to compensate the loss of pollen in so many ways, the anthers produce a far larger amount than is necessary for the fertilisation of the san1e flower. I know this from my own experiments on Ipomooa, given in the Introduction; aud it is still more plainly shown by the astonishingly small quantity produced by cleistogene flowers, which lose none of their pollen, in comparison with that produced by the open flowers borne by the same plants~ and yet this small quantit)· suffices for the fertilisation of all their numerous seeJs. Mr. Hassall took pains in estimating tho ~lumber of pollen-grains produced by a flower of the Dandelion (Leontodon), and found the number to be 243,600, and in a Preony 3,654,000 grains.t The editor of the 'Botanical Register' counted the ovules in the flowers of Wistaria sinensis, and carefully estimated * Mr. ·Blackley observed that the ripe anthers of rve did not dehisce whilst kept under a bellglass in a damp atmosphere, whilst other anthers exposed to the same tempvrature in the open air dehisced freely. He also found much more pollen adhering to the sticky sliJes, which were attached to kites and sent high up in tl1e atmosphere, during the fir:st fine and dry days after wet weather, tllan ut other times: ' Experimental Uesl:'arche::i on Hay F ever,' 1873, p. 127. t ' Die Schutzmittel des Pollens,' 187:-3. t 'Annals and 1\iag. of Nat. Hist.' \'Ol. viii. 184:2, p. 108. ()HAP . .X. MEANS OF CROSS-I!'ERTII-ISATION. 377 the number of pollen g1·ains, and he found that for each ovule there were 7,00J (~rains.* With 1\iirabilis, thre6 or four of the very large pollen-grains are sufficient to fertilise an ovule; bnt [ do not know how many grains a flower produces. \ \ i ~h Hibiscus, Kolreuter found that sixty grains wore necessary to fertilise all the ovules of a flower, and .he calculated that 4863 grains \Yere produced ~y a Single flower, or eighty-one times too m~ny. With Geum. urbanum, however, according to Gartner, the pollen Is only ten times too much.f 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 o~en? .As ~any 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 m~y be seen at the present time in Lathyrus nissolia, Bwphytum sensitivurn, and several other plants. The answer to the above question obviously is, that w'ith perJ1?.anently closed flowers there could be no crossfertilisation. . The frequency, almost regularity, with whi0h pollen Is transported by insects from flower to flower often fron1 a considerable distance, well deserves atte~tion.i • Quoted in 'Gard. Chron.' 1 846, p. 771. . t Jfolreuter, 'V01·laufige Nach- 1tcht, 1761, p. 9. Gartuer, 'Beitrage zur Kenntniss,' &c. p. 346. t An exreriment made by Kolreuter (' Fortset:suug,' &c. 17€3, p .. 69) affo1·ds good evidence on this head. Hibi;;ous vesicarius is bt~ongly dichogumous, its pollen ~mg shed before the stigmas are matnre. Kolreuter marked 31() flowers, and put pollen from other flowers on their stigrrws every dAy, so that they were thoroughly fertilised; and he left the same number of other Bm-ver::; to the ngcncy of insects. Afterwards he counted the seerls of both lots: the flowers which h had fertilisell with such a:;tonishing care produced 11,2i37 seeds, |