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Show 382 CROSS-FERTILISATION. CHAP. X. an incidental result, or has not been specially acquired for the sake of preventing self-fertibsation. On the other hand, there can hardly be a doubt that dichogamy, which prevails according to Ifildebrand * in the ~?eater. number of species,-that the heterostyled conchtwn of certain plants,--and that many mechanical structures -have all been acquired so as both to check selffertilisation and to favour cross-fertilisation. The means for favouring cross-fertilisation must have been acquired before those which prevent self-fertilisation; as it would manifestly be injurious to a plant that its stigma should fail to receive its own poll: n, unless it had already become well adapted for recenring pollen from a~other individual. It should be observed that many plants still possess a high power of self-fertilisation, although their flowers are excellently constructed for cross-fertilisation- for instance, those of many papilionaceous species. It may be admitted as aln1ost certain that some structures, such as a narrow elongated nectary, or a lono- tubular corolla, have been developed in order 5 . h that certain kinds of insects alone should obtmn t e nectar. These insects would thus find a store of nectar preserved from the attacks of other insects ; and they would thus be led to visit frequently such flowers and to carry pollen froin one to the other.t It might perhaps have been expected that plants having their flowers thus peculiar! y constructed would pro:fi t in a greater degree by being crossed, than ordinary or simple flowers; but this does not seem to hold good. Thus Tropmolu1n minus has a long nectary and an irregular corolla, whilst Limnanthes "~< ' Die Geschlc.chter Vertheiluug,' &c. p. 32. ;. t See the interesting discussion nn this subject by H. Miiller, • Die Befrud1tnng.' &c. P· 431. CHAP. X. INCONSPICUOUS FLOWERS. 383 ~ouglasii ~as a regula_r flower and no proper nectary, ) et the CI ossecl seedlings of both species are t th self-fertilised in height as 100 to 79 Salvll·a c 0 . · · e ~ occttnea ha~ an. Irregular corolla, vvi th a curious apparatus by which Insects depress the stamens, while the flowers of I pomcea are re?'ular : and the crossed seedlings of the former are In height to the self-fertilised as 100 to 76, whilst thos~ of the Ipomcea are as 100 to 77. ~agopy:·um is dimorphic, and Anagallis collina is ~on-~Imorphic, and the crossed seedlings of both are In height to the self-fertilised as 100 to 69. . With all European. plants, excepting the comparati. ve!y ra~·e . a~emophilous kinds, the possibility of d~s~Inct I~dividuals intercTossing depends on the VISits of Insects;. and H. Muller has proved by his valu~b!e obseTvations, that large conspicuous flowers a:e VIsite~ much more frequently and by many more kinds of Insects, than are small inconspicuous flowers. ~e. further remarks that the flowers which are rarely . VISited must be capable of self-fertilisation, otherwise they would quickly become extinct.* There is however, .some liability to error in forming a jud~ment ?n tlus head, from the extreme difficulty of ascertainIng. whether flowers which are Tarely or never visited during th~ day (as in the above given case of Fumaria capreolata) are not visited by sinall nocturnal Lepidoptera, which are known to be strongly attracted by su?ar. t The two lists given in the early part of this ~hapter support Muller's conclusion that small and · Inconspicuous flowers are completely self-fertile ; , * 'Befruchtung,' &c., p. 426. Nature,' 1873, p. 433. t In answer to a question by ~e, the editor of an entomoloo-ical Jou_rnal writes- ,, The Depores-saru: e ' as 1· s no t on.o us to every collertor of N octuro1 come very freely to sugar1 and no doubt naturally visit flowers:" the 'Entomologists' Weekly In telligencer,' 1860, p. 103. |