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Show 342 CONCLUDING REMARKS CHAP. VIII. two states on the same plant, in Viola, Oxalis, Biophytum, Ot:unpanula, &c. In the several species of Viola the various parts of the flowers have also been 1noclified in very different degrees. Those plants which in their own country produce flowers of full or nearly full size, but never expand (as with Thelymitra ), and yet set fruit, might easily be rendered cleistogamic. Lathyrus nissolia seems to be in an incipient transitional state, as does Drosera An,qlica, the flowers of which are not perfectly closed. There is good evidence that flowers so1netimes fail to expand and are somewhat reduced in size, owing to exposure to unfavourable conditions, but still retain their fertility uni1npaired. Linnceus observed in 1753 that the flowers on several plants brought fro1n Spain and grown at Upsala did not show any corolla and yet produced seeds. Asa Gray has seen flo~ers on exotic plants in the Northern United States which never expanded and yet fruited. With certain English plants, which bear flowers during nearly the whole year, Mr. Bennett found that those produced during the winter season were fertilised in the bud; whilst with other species having fixed times for flowering, but "which had been tempted by a mild January to put forth a few wretched flowers," no pollen was discharged from the anthers, and no seed was formed. The flowers of Lysirnachia vulgaris if fully exposed to the sun expand properly, while those growing in shady ditches have smaller corollas which open only slightly; and these two forms graduate into one another in intermediate stations. Herr Bouche's observations are of especial interest, for he shows that both temperature and the an1ount of light affect the size of the corolla; and he gives measurements proving that with some plants the corolla is diminished by the increasing cold and CHAP. VIII. ON CLEISTOGAMIC FLOWERS. 34 ~a~kn~ss. o! the changing season, whilst with oth r It IS dimi~Ished by the increasing heat and light* The behef that the first step towar·'ls flo b. . d d . . " wers e1ncr ren. ere cleistob<Yamic was due to the conel 1't ·I ons tbo whic~. they were expos~d, is supported by the fact of vanous plants belongin<Y to this class e1' th t . d · h . o er no pro- _ucing t eu cleistogamic flowers under certain concli-tions, or, on the other hand, producing them to th comp. letef evx· clusion of the perfect ones. Thu s some spec1~s o 1ola do not bear cleistogamic flowers when growing on the l?wlands or in certain districts. Other plants whe~ cultivated have failed to produce perfect flowers d ur.Ing several successive years ; and this is the ~ase wit~ Juncus bufonius in its native land of Rus~Ia. Oleistogamic flowers are produced by some species late and by others early in the season · and th' . h h . ' IS agrees wit t e view that the first step towards their de-vel? pment was due to climate; though the periods at which the two sorts of flowers now appear must since have become much more distinctly defined. We do not know wheth.er too low. or too high a temperature or the amount of hght acts In a direct manner on the size of the corolla, or indirectly thr~ugh the male organs being first affecte~. However this may be, if a plant were prevent.ed e.Ither early o~ late in the season from fully expan.ding 1 ts corolla, w1 th some reduction in its size, but With no loss of the power of self-fertilisation then natural selection might well complete the work and * For the statement by Linnrous see Mohl in' Bot. Zeitung' 1863, p. 327. Asa Gray, 'A~erica~ Journal of Science' 2nd series yol. xxxix. 1865, p. l05. Bennett m 'Nature,' Nov. 1869, p. 11. The Rev. G. Henslow also says ('Gardener's Chronicle' 1877 p 271 : also 'Nature,' oct. 19, 1S76: P· 543) '' that when the autumn draws on, and habitually in winter for such of our wild :flowers as blossom at t~a~ season," the flowers ar~ self-fertlltsed. Ou Lysimachl~, H. Muller, 'Nature,' Sept. 187.3, p. 433. Bouche, 'Sitzungsbencht der Gesell. N aturforsch. Freunde,' Oct. 1874, p. 90. |