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Show 340 CONCLUDING REMARKS CHAP. VIII. flowers being irregular, one genus alone ~ut of the 38 heterosty led genera described in the previous chapters b ear.s sue h flowers·, yet all these gen.e ra arfe ab.solutely 1 . . dependent on insects for their legi~1m~te _ert1 1~atwn. I know not how to account for th1s d1ffer en?e 1n the · proportion of the plants bearing ~egular and uregular flowers in the two classes, unless 1t be that the heterostyled flowers are already so well adapt~d for cross-fer-t l. l.l Sat 'lO ll, through the position of then stam. ens and pistils and the difference in power of then two. or three kinds of pollen, that any additional a~aptatwn, nainely, through the flowers being made uregular, has been rendered superfluous. . . Although cleistogamic flowers never fa1l . to y1eld •1 large number of seeds, yet the plants beanng them ~sually produce perfect flowers, either _simultaneously or more co1n1nonly at a different pe:1?d ;. and these are adapted for or admit of cross-fert1hs~t1on. F:om the cases given of the two Indian .species of Vwl~, which produced in this country dunng several years only cleistogamic flowers, and of the numero~s pla~ts of Vandellia and of some plants of Ononis wluch behaved during one whole season in the same manner, it appears rash to infer froin such cases as that of Salvia cleistogama not having produced perfect ~owers during five years in Germany,* and of ~n As~ICar~ not having done so during several years In Pa~Is, th. these plants would not bear perfect flowers In t~eu native homes. Von Mohl and several other botanists have repeatedly insisted that as a general rule the perfect flowers produce d b y c1 e 1· s t og.a mi· c plants ahre Sterile· but it has been shown under the head of; et several' species that this is not the case. The per1ec * Dr. Ascherson, 'Bot. Zeit.' 1871, P· 555· CHAP. VIII. ON CLEISTOGAMIC FLOWERS. 341 flowers of Viola are indeed sterile unless th y are visited by bees ; but when thus visited th y yi ld. the full number of seeds. As far as I have been abl to discover there is only one absolute exception t the rule that the perfect flowers are fertile, namely, that of Voandzeia; and in this case we should remember that cultivation often affects injuriously the reproductive organs. Although the perfect flowers f Leersia sometimes yield seeds, yet this occurs so rarely, as far as hitherto observed, that it practically forms a second exception to the rule. As cleistogamic flowers are invariably fertilised, and as they are produced in large numbers, they yield. altogether a much larger supply of seeds than do the perfect flowers on the same plant. But the latter flowers will occasionally be cross-fertilised, and their offspring will thus be invigorated, as we may infer from a wide-spread analogy. But of such invigoration I have only a small amount of direct evidence : two crossed seedlings of Ononis minutissima were put into competition with two seedlings raised from cleistogamic flowers ; they were at first all of equal height ; the crossed were then slightly beaten ; but on the following year they showed the usual superiority of their class, and were to the self-fertilised plants of cleistogamic origin as 100 to 88 in mean height. With Vanclellia twenty crossed plants exceeded in height twenty plants raised fro1n cleistogamic seeds only by a little, namely, in the ratio of 100 to 94. It is a natural inquiry how so many plants belonging to various very distinct families first came to have the development of their flowers arrested, so as ultimately to become cleistogamic. That a passage from the one state to the other is far from difficult is shown by the many recorded cases of gradations between the |
