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Show CONCLUDING REMARKS. CHAP. VIII. to prevent the fertilisation of the perfect flowers. I do not doubt that this holds good to a certainli1nitedextent, but the production of a large supply of seeds with little consumption of nutrient matter or expenditure of vital force is probably a far more efficient motive power. The whole flower is much reduced in size; but what is much more i1nportant, an extremely small quantity of pollen has to be formed, as none is lost through the action of insects or the weather; and pollen contains 1nuch nitrogen and phosphorus. Von Mohl esti1nated that a single cleistogamic anther-cell of Oxalis acetasella contained from one to two dozen pollen -grains ; we will say 20, and if so the whole flower can have produced at most 400 grains; with Impatiens the whole number may be estimated in the sa1ne manner at 250 ; with Leersia at 210 ; and with Viola nana at only 100. These figures are wonderfully low compared with the 243,600 pollen-grains produced by a flower of Leontodon, the 4,863 by an Hibiscus, or the 3,654,000 by a Preony.* We thus see that cleistogamic flowers produce seeds with a wonderfully small expenditure of pollen ; and they produce as a general rule quite as many seeds as the perfect flowers. . · That the production of a large nun1ber of seeds ~s necessary or beneficial to many plants needs no evidence. So of course is their preservation before they are ready for gennination; and it is one of the many remarkable peculiarities of the plants which bear cleistogamic flowers, that an incomparably larger pr~portion of them than of ordinary plants bury. the~r young ovaries in the ground ;-an action whwh. 1t may be presumed serves to protect them from bemg *· The authorities for these statements are g1· ven m· my 'Effects of C!'Css and Self-Fertilisation,' p. ::376. CnAP. VIII. ON CLEISTOG.Al\1IC FLO,VER . 339 devol~red by birds or other enemies. But this ath-ant~ ge 1s. ac~ompanied by the loss of the pow r of wide d· issehm in·a tion. No le.s s . than eio·ht of th .. b e gon 1 a, 1n t. e hst .at the beginning of this chapter inclu le s~ec1es w~1ch act in this manner, namely, ~ev ral kinds of V 1ola, Oxalis, V andellia, Linaria, Con1molina, and at least three genera of Leguminosm. The seeds also of Leersia, though not buried, arc concealed in the most perfect manner within tho sheaths of tho leaves. .Cleisto?amic flowers possess great facilities for .burying t~eu Y?ung ovaries or capsules, owing to then small s1zo, po1nted shape, closed condition and the absence of a corolla; and we can thus understand how it is that so many of them have acquired this curious habit. It has already been shown that in about 32 out of the 55 gen~ra in the list just referred to, the perfect flowers are. uregular; and this implies that they have been specially adapted for fertilisation by insects. Moreover three of the genera with regular flowers are adapted by other means for the same end. Flowers thus. constructed are liable during certain seasons to ?e Imperfectly fertilised, namely, when the proper Insects are scarce; and it is difficult to avoid the belief that the production of cleisto()'amic flowers wh I'C h ensures und~r all circu1nstances b a full supply' of seed, h~s b~en In part determined by the perfect ~owers being hable to fail in their fertilisation. But 1f this determining cause be a real one it must be of s~bordinate .i~portance, as four of the 'genera in the hst are fertilised by the wind ; and there seems no , reason why their perfect flowers should fail to be fertilised. more frequently than those in any other anemoph1lous genus. In contrast with what we here see with respect to the large proportion of the perfect z 2 |
