OCR Text |
Show 370 MEANS OF CROSS-FERTILISATION. CHAP. X. Each of these lists contains by a mere accident the san1e number of genera, viz., forty-nine. The genera in the first list include sixty-:five species, and those in the second sixty species ; the Orchidem in both being t~xcluded. If the genera in this latter order, as well as in the Asclepiadro and Ap~cynacero, hE~d b~e~ included, the number of species which are stenle If Insects are excluded would have been greatly increased; but the lists are confined to species which were actually experiinented on. The results can be considered as only approximately accurate, for fertility is so variable. a eharacter, that each species ought to have been tned 1nany times. The above number o~ ~pecies, namely, 125 is as nothing to the host of hving plants; but the 'mer.e fact of more than half of them being sterile within the specified degree, when insects are exclud~d, is a striking one; for whenever pollen has to be earned from the anthers to the stigma in order to ensure full fertility, there is at least a good chance of cross-fertili~ ation. I do not, however, believe that if all known }>lants were tried in the Saine mann ~·, half, W?uld be found to be sterile within the specified hm1ts; for 1nany flowers were s lected for experiment which pre· sented some remarkable structure; and such flowers often require insect-aid. Thus out of the forty-nine genera in the first list, about thirty-two have flowers which are asymn1etrical or present some remarka~le peculiarity; whilst in the second l~st, inclu~ing spec1~: which are fully or moderately fertile when Insects w~I excluded, only about twenty-one out of the forttn.~ne are asymmetrical or present any remarkable pecuhan Y· Means of cross-fertilisation.-The most .important of a] l the means by which pollen is earned from the anthers to the stigma of the same flower, or from flower CHAP. X. MEANS OF CROSS-FERTILISATION. 371 to flower, are insects, belonging to the orders of Hymenoptera, Lepidoptera, and Diptera ; and in some parts of the world, birds.* Next in importance, but in a quite subordinate degree, is the wind; and with some aquatic plants, according to Delpino, currents of water. The simple fact of the necessity in many cases of extraneous aid for the transport of the pollen, and the many contrivances for this purpose, render it highly probable that some great benefit is thus gained; an(l this conclusion has now been :finnly established by the proved superiority in growth, vigour, and fertility of plants of crossed parentage over those of self-fertilised parentage. But we should always keep in mind that two somewhat opposed ends have to be gvctined; the first and more important one being the proclaotion * I will here give all the cases known to me of birds fcrtilisinoflowers. In South Brazil, hun~ ming-birds certainly fertilise the various species of Abutilon, which are sterile without their nid: (F1'itz· Miiller, ' Jenaische Zeitschrift f. Naturwiss.' B. vii. 1872, p. 24.) Long-beaked humming-birds v.i:s:lti the flowers of Brugmansia, whilst some of the shol't-beaked species often penetrate its large C0i'Glla in Ol'der to obtain the nectnr in an illegitimate manner, in the sn~e · manner as do bees in aU parts of the world. It appt-ars, iudeed~ that the beaks of bummin<Y-Lhds a!e specially n.claptcd. to tho va-. r~o~s kinds of fiowers which they VIsit: on the Cordillera il1ey surk the Salviro,. and. laoerate the flowers of the Tac.soniro; in Nicaragua, M:~:. Belt saw tl1em snckinU' the fio.wers of Mm:cgra..via apd Erythrma, and thus they carried pollen from flower to. flower; In North Amel'ica they are said to frequent the tlowers of Impatiens: (Gould, 'Introduction to the Tro. chiliclm/ 1861, pp. 15, 120; ' Garu. Chron.iclo; 1869, p. 389; 'The Natural'i::.t in Nicaragua,' p. 129; (,Journ"l of Linn. Soc. Bot.' vol. xiii. 1872, p. 151.) I may add that I oftc~n saw in Chile a Mimm; with its head yellow with. pollen from, as I believe, a Cassia. I hHve been assured that at the Cape of Good Hope, Stre·litzi~t it:l iertilisecl by tl1e N ectarinidre. There can hard1y he a· doul'>t that ma,ny Australian. flowers are fertilised by the many honey-sucking birds of that country. Mr. vV:~Jhwe remar·ks (address to the B10~ logical Section, Brit. Assoc. 1876) that h.e has "often observed the beaks and, faces of the brushtongued lories of the Moluccas covered wjth pollen." · In Now ~E:alaucl, man . y specimens of tlie Anthornis mela1nwa ha..d th eir .beads ooloured with pollen from the flowers of au endemic specie.' of Fuohsi£1: (Potts, ' Transact. N.ew Zealand; Institute,' vol. iii. 1870, p. 72.) 2 ll 2 |