OCR Text |
Show 1 MEANS OF CROSS-FERTILISATION. CHAP. X. curious fact that certain individuals of the n1oncecious walnut (Juglans regia) are proterandrous, ancl others proterogynous, and these will reciprocally fertilise each other. So it is with the co1nmon nut ( Oorylus avellana), * and, what is n1ore surprising, ·with so1ne few hernlaphrodite plants, as observed by H. Mii.ller.t Those latter plants cannot fail to act on each other like dimorphic or tri1norphic species, in which tho union of two individuals is necessary for full. and normal f rtility. 'iVith ordinary hennaphrodite species, the expansion of only a few flowers at the same time is one of the simplest means for favouring the intercrossing of distinct individuals; but this would render the plantR less conspicuous to insects, unless the flowers wore of large size, as in the case of several bulbous plants. Kerner thinks t that it is for this obj ct that the Australian Villarsia parnassifolia produces daily only a single flower. Mr. Cheeseman also remarks,§ that as certain Orchids in New Zealand which require insect-aid for their fertilisation bear only a single flower, distinct plants cannot fail to intercross. Dichogamy, which prevails so extensively throughout the vegetable kingdom, much increases the chance of distinct individuals intercrossing. With protorandrous species, which are far n1or common than proterogynous, the young flowers arc exclusively male in function, and the older ones e.~ elusively female; and as bees habitually alight low down on the spikeR of flowers in order to crawl upwards, they got dusted with pollen from the uppermost flowers, which they carry to the stigmas of the lower and older flowers on the next spike which they visit. The degree to * 'Nnturo,' 1875, p. 26. t 'Die Befruchtllng,' &c. pp. 2~5 , 339. + 'Die Sc·hutzmittel,' &c. P· 23. § 'Transact. New Zeahmd Institute,' v.ol. v. 1873, p. 356. CHAP. X. PREPOTENT POLLEN. 3Ul which distinct plant~ wil~ thus be intercrossed depends on the number of sp1kes 1n full flower at the same time on the same plant. With proterogynous flowers and with depending racemes, the 1nanner in which insects visit the flowers ought to be reversed in order that distinct plants should be intercrossed. But this whole ~ubject requires further investigation, as the great Importance of crosses between distinct individuals instead of merely between distinct flowers, has hitherto' been hardly recognised. In some few cases the special movements of certain organs almost ensure pollen being carried from plant to plant. Thus with many orchids, the pollen-masses after becoming attached to the head or proboscis of an insect do not move into the proper position for strikin<Y the stigma, until ample time has elapsed for the insec~ to fly to another plant. With 'Spiranthes autumnalis, the pollen-masses cannot be applied to the stigma until the labellum and rostellum have moved apart, and this movement is very slow.* With Posoqueria Jragrans (one of the Rubiacere) the same end is gained by the movement of a specially constructed stamen, as described by Fritz Muller. We now come to a far more general and therefor more important means by which the mutual fertilisation of distinct plants is effected, namely, the fertilising po~er of pollen from another variety or individual h.mng greater than that of a plant's own pollen. The ~1mplest and best known case of prepotent action In pollen, though it does not bear directly on our present subject, is that of a plant's own pollen over that from a distinct species. If pollen from a distinct species he placed on the stigma of a castrated flower, and then b"' ''~he Va~·i?us Contrivances Orchids are fertilised,' 1st_ edit. Y WhiCh .Bntish and ]'oreign p. J 28. |