MEANS OF CROSS-FERTILISATION.
The most important of all the means by which pollen is carried from the anthers to the stigma of the same flower, or from flower to flower, are insects, belonging to the orders of Hymenoptera, Lepidoptera, and Diptera; and in some parts of the world, birds. (10/1. I will here give all the cases known to me of birds fertilising flowers. In South Brazil, humming-birds certainly fertilise the various species of Abutilon, which are sterile without their aid (Fritz Muller ‘Jenaische Zeitschrift f. Naturwiss.’ B. 7 1872 page 24.) Long-beaked humming-birds visit the flowers of Brugmansia, whilst some of the short-beaked species often penetrate its large corolla in order to obtain the nectar in an illegitimate manner, in the same manner as do bees in all parts of the world. It appears, indeed, that the beaks of humming-birds are specially adapted to the various kinds of flowers which they visit: on the Cordillera they suck the Salviae, and lacerate the flowers of the Tacsoniae; in Nicaragua, Mr. Belt saw them sucking the flowers of Marcgravia and Erythina, and thus they carried pollen from flower to flower. In North America they are said to frequent the flowers of Impatiens: (Gould ‘Introduction to the Trochilidae’ 1861 pages 15, 120; ‘Gardeners’ Chronicle’ 1869 page 389; ‘The Naturalist in Nicaragua’ page 129; ‘Journal of the Linnean Society Botany’ volume 13 1872 page 151.) I may add that I often saw in Chile a Mimus with its head yellow with pollen from, as I believe, a Cassia. I have been assured that at the Cape of Good Hope, Strelitzia is fertilised by the Nectarinidae. There can hardly be a doubt that many Australian flowers are fertilised by the many honey-sucking birds of that country. Mr. Wallace remarks (address to the Biological Section, British Association 1876) that he has “often observed the beaks and faces of the brush-tongued lories of the Moluccas covered with pollen.” In New Zealand, many specimens of the Anthornis melanura had their heads coloured with pollen from the flowers of an endemic species of Fuchsia (Potts ‘Transactions of the New Zealand Institute’ volume 3 1870 page 72.) 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; and this conclusion has now been firmly 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 gained; the first and more important one being the production of seeds by any means, and the second, cross-fertilisation.
The advantages derived from cross-fertilisation throw a flood of light on most of the chief characters of flowers. We can thus understand their large size and bright colours, and in some cases the bright tints of the adjoining parts, such as the peduncles, bracteae, etc. By this means they are rendered conspicuous to insects, on the same principle that almost every fruit which is devoured by birds presents a strong contrast in colour with the green foliage, in order that it may be seen, and its seeds freely disseminated. With some flowers conspicuousness is gained at the expense even of the reproductive organs, as with the ray-florets of many Compositae, the exterior flowers of Hydrangea, and the terminal flowers of the Feather-hyacinth or Muscari. There is also reason to believe, and this was the opinion of Sprengel, that flowers differ in colour in accordance with the kinds of insects which frequent them.
Not only do the bright colours of flowers serve to attract insects, but dark-coloured streaks and marks are often present, which Sprengel long ago maintained served as guides to the nectary. These marks follow the veins in the petals, or lie between them. They may occur on only one, or on all excepting one or more of the upper or lower petals; or they may form a dark ring round the tubular part of the corolla, or be confined to the lips of an irregular flower. In the white varieties of many flowers, such as of Digitalis purpurea, Antirrhinum majus, several species of Dianthus, Phlox, Myosotis, Rhododendron, Pelargonium, Primula and Petunia, the marks generally persist, whilst the rest of the corolla has become of a pure white; but this may be due merely to their colour being more intense and thus less readily obliterated. Sprengel’s notion of the use of these marks as guides appeared to me for a long time fanciful; for insects, without such aid, readily discover and bite holes through the nectary from the outside. They also discover the minute nectar-secreting glands on the stipules and leaves of certain plants. Moreover, some few plants, such as certain poppies, which are not nectariferous, have guiding marks; but we might perhaps expect that some few plants would retain traces of a former nectariferous condition. On the other hand, these marks are much more common on asymmetrical flowers, the entrance into which would be apt to puzzle insects, than on regular flowers. Sir J. Lubbock has also proved that bees readily distinguish colours, and that they lose much time if the position of honey which they have once visited be in the least changed. (10/2. ‘British Wild Flowers in relation to Insects’ 1875 page 44.) The following case affords, I think, the best evidence that these marks have really been developed in correlation with the nectary. The two upper petals of the common Pelargonium are thus marked near their bases; and I have repeatedly observed that when the flowers vary so as to become peloric or regular, they lose their nectaries and at the same time the dark marks. When the nectary is only partially aborted, only one of the upper petals loses its mark. Therefore the nectary and these marks clearly stand in some sort of close relation to one another; and the simplest view is that they were developed together for a special purpose; the only conceivable one being that the marks serve as a guide to the nectary. It is, however, evident from what has been already said, that insects could discover the nectar without the aid of guiding marks. They are of service to the plant, only by aiding insects to visit and suck a greater number of flowers within a given time than would otherwise be possible; and thus there will be a better chance of fertilisation by pollen brought from a distinct plant, and this we know is of paramount importance.
The odours emitted by flowers attract insects, as I have observed in the case of plants covered by a muslin net. Nageli affixed artificial flowers to branches, scenting some with essential oils and leaving others unscented; and insects were attracted to the former in an unmistakable manner. (10/3. ‘Enstehung etc. der Naturhist. Art.’ 1865 page 23.) Not a few flowers are both conspicuous and odoriferous. Of all colours, white is the prevailing one; and of white flowers a considerably larger proportion smell sweetly than of any other colour, namely, 14.6 per cent; of red, only 8.2 per cent are odoriferous. (10/4. The colours and odours of the flowers of 4200 species have been tabulated by Landgrabe and by Schubler and Kohler. I have not seen their original works, but a very full abstract is given in Loudon’s ‘Gardeners’ Magazine’ volume 13 1837 page 367.) The fact of a larger proportion of white flowers smelling sweetly may depend in part on those which are fertilised by moths requiring the double aid of conspicuousness in the dusk and of odour. So great is the economy of nature, that most flowers which are fertilised by crepuscular or nocturnal insects emit their odour chiefly or exclusively in the evening. Some flowers, however, which are highly odoriferous depend solely on this quality for their fertilisation, such as the night-flowering stock (Hesperis) and some species of Daphne; and these present the rare case of flowers which are fertilised by insects being obscurely coloured.
The storage of a supply of nectar in a protected place is manifestly connected with the visits of insects. So is the position which the stamens and pistils occupy, either permanently or at the proper period through their own movements; for when mature they invariably stand in the pathway leading to the nectary. The shape of the nectary and of the adjoining parts are likewise related to the particular kinds of insects which habitually visit the flowers; this has been well shown by Hermann Muller by his comparison of lowland species which are chiefly visited by bees, with alpine species belonging to the same genera which are visited by butterflies. (10/5. ‘Nature’ 1874 page 110, 1875 page 190, 1876 pages 210, 289.) Flowers may also be adapted to certain kinds of insects, by secreting nectar particularly attractive to them, and unattractive to other kinds; of which fact Epipactis latifolia offers the most striking instance known to me, as it is visited exclusively by wasps. Structures also exist, such as the hairs within the corolla of the fox glove (Digitalis), which apparently serve to exclude insects that are not well fitted to bring pollen from one flower to another. (10/6. Belt ‘The Naturalist in Nicaragua’ 1874 page 132.) I need say nothing here of the endless contrivances, such as the viscid glands attached to the pollen-masses of the Orchideae and Asclepiadae, or the viscid or roughened state of the pollen-grains of many plants, or the irritability of their stamens which move when touched by insects etc.—as all these contrivances evidently favour or ensure cross-fertilisation.
All ordinary flowers are so far open that insects can force an entrance into them, notwithstanding that some, like the Snapdragon (Antirrhinum), various Papilionaceous and Fumariaceous flowers, are in appearance closed. It cannot be maintained that their openness is necessary for fertility, as cleistogene flowers which are permanently closed yield a full complement of seeds. Pollen contains much nitrogen and phosphorus—the two most precious of all the elements for the growth of plants—but in the case of most open flowers, a large quantity of pollen is consumed by pollen-devouring insects, and a large quantity is destroyed during long-continued rain. With many plants this latter evil is guarded against, as far as is possible, by the anthers opening only during dry weather (10/7. Mr. Blackley observed that the ripe anthers of rye did not dehisce whilst kept under a bell-glass in a damp atmosphere, whilst other anthers exposed to the same temperature in the open air dehisced freely. He also found much more pollen adhering to the sticky slides, which were attached to kites and sent high up in the atmosphere, during the first fine and dry days after wet weather, than at other times: ‘Experimental Researches on Hay Fever’ 1873 page 127.)—by the position and form of some or all of the petals,—by the presence of hairs, etc., and as Kerner has shown in his interesting essay, by the movements of the petals or of the whole flower during cold and wet weather. (10/8. ‘Die Schutzmittel des Pollens’ 1873.) In order to compensate the loss of pollen in so many ways, the anthers produce a far larger amount than is necessary for the fertilisation of the same flower. I know this from my own experiments on Ipomoea, given in the Introduction; and it is still more plainly shown by the astonishingly small quantity produced by cleistogene flowers, which lose none of their pollen, in comparison with that produced by the open flowers borne by the same plants; and yet this small quantity suffices for the fertilisation of all their numerous seeds. Mr. Hassall took pains in estimating the number of pollen-grains produced by a flower of the Dandelion (Leontodon), and found the number to be 243,600, and in a Paeony 3,654,000 grains. (10/9. ‘Annals and Magazine of Natural History’ volume 8 1842 page 108.) The editor of the ‘Botanical Register’ counted the ovules in the flowers of Wistaria sinensis, and carefully estimated the number of pollen-grains, and he found that for each ovule there were 7000 grains. (10/10. Quoted in ‘Gardeners’ Chronicle’ 1846 page 771.) With Mirabilis, three or four of the very large pollen-grains are sufficient to fertilise an ovule; but I do not know how many grains a flower produces. With Hibiscus, Kolreuter found that sixty grains were necessary to fertilise all the ovules of a flower, and he calculated that 4863 grains were produced by a single flower, or eighty-one times too many. With Geum urbanum, however, according to Gartner, the pollen is only ten times too much. (10/11. Kolreuter ‘Vorlaufige Nachricht’ 1761 page 9. Gartner ‘Beitrage zur Kenntniss’ etc. page 346.) As we thus see that the open state of all ordinary flowers, and the consequent loss of much pollen, necessitate the development of so prodigious an excess of this precious substance, why, it may be asked, are flowers always left open? As many plants exist throughout the vegetable kingdom which bear cleistogene flowers, there can hardly be a doubt that all open flowers might easily have been converted into closed ones. The graduated steps by which this process could have been effected may be seen at the present time in Lathyrus nissolia, Biophytum sensitivum, and several other plants. The answer to the above question obviously is, that with permanently closed flowers there could be no cross-fertilisation.
The frequency, almost regularity, with which pollen is transported by insects from flower to flower, often from a considerable distance, well deserves attention. (10/12. An experiment made by Kolreuter ‘Forsetsung’ etc. 1763 page 69, affords good evidence on this head. Hibiscus vesicarius is strongly dichogamous, its pollen being shed before the stigmas are mature. Kolreuter marked 310 flowers, and put pollen from other flowers on their stigmas every day, so that they were thoroughly fertilised; and he left the same number of other flowers to the agency of insects. Afterwards he counted the seeds of both lots: the flowers which he had fertilised with such astonishing care produced 11,237 seeds, whilst those left to the insects produced 10,886; that is, a less number by only 351; and this small inferiority is fully accounted for by the insects not having worked during some days, when the weather was cold with continued rain.) This is best shown by the impossibility in many cases of raising two varieties of the same species pure, if they grow at all near together; but to this subject I shall presently return; also by the many cases of hybrids which have appeared spontaneously both in gardens and a state of nature. With respect to the distance from which pollen is often brought, no one who has had any experience would expect to obtain pure cabbage-seed, for instance, if a plant of another variety grew within two or three hundred yards. An accurate observer, the late Mr. Masters of Canterbury, assured me that he once had his whole stock of seeds “seriously affected with purple bastards,” by some plants of purple kale which flowered in a cottager’s garden at the distance of half a mile; no other plant of this variety growing any nearer. (10/13. Mr. W.C. Marshall caught no less than seven specimens of a moth (Cucullia umbratica) with the pollinia of the butterfly-orchis (Habenaria chlorantha) sticking to their eyes, and, therefore, in the proper position for fertilising the flowers of this species, on an island in Derwentwater, at the distance of half a mile from any place where this plant grew: ‘Nature’ 1872 page 393.) But the most striking case which has been recorded is that by M. Godron, who shows by the nature of the hybrids produced that Primula grandiflora must have been crossed with pollen brought by bees from P. officinalis, growing at the distance of above two kilometres, or of about one English mile and a quarter. (10/14. ‘Revue des Sc. Nat.’ 1875 page 331.)
All those who have long attended to hybridisation, insist in the strongest terms on the liability of castrated flowers to be fertilised by pollen brought from distant plants of the same species. (10/15. See, for instance, the remarks by Herbert ‘Amaryllidaceae’ 1837 page 349. Also Gartner’s strong expressions on this subject in his ‘Bastarderzeugung’ 1849 page 670 and ‘Kenntniss der Befruchtung’ 1844 pages 510, 573. Also Lecoq ‘De la Fecondation’ etc. 1845 page 27. Some statements have been published during late years of the extraordinary tendency of hybrid plants to revert to their parent forms; but as it is not said how the flowers were protected from insects, it may be suspected that they were often fertilised with pollen brought from a distance from the parent-species.) The following case shows this in the clearest manner: Gartner, before he had gained much experience, castrated and fertilised 520 flowers on various species with pollen of other genera or other species, but left them unprotected; for, as he says, he thought it a laughable idea that pollen should be brought from flowers of the same species, none of which grew nearer than between 500 and 600 yards. (10/16. ‘Kenntniss der Befruchtung’ pages 539, 550, 575, 576.) The result was that 289 of these 520 flowers yielded no seed, or none that germinated; the seed of 29 flowers produced hybrids, such as might have been expected from the nature of the pollen employed; and lastly, the seed of the remaining 202 flowers produced perfectly pure plants, so that these flowers must have been fertilised by pollen brought by insects from a distance of between 500 and 600 yards. (10/17. Henschel’s experiments quoted by Gartner ‘Kenntniss’ etc. page 574, which are worthless in all other respects, likewise show how largely flowers are intercrossed by insects. He castrated many flowers on thirty-seven species, belonging to twenty-two genera, and put on their stigmas either no pollen, or pollen from distinct genera, yet they all seeded, and all the seedlings raised from them were of course pure.) It is of course possible that some of these 202 flowers might have been fertilised by pollen left accidentally in them when they were castrated; but to show how improbable this is, I may add that Gartner, during the next eighteen years, castrated no less than 8042 flowers and hybridised them in a closed room; and the seeds from only seventy of these, that is considerably less than 1 per cent, produced pure or unhybridised offspring. (10/18. ‘Kenntniss’ etc. pages 555, 576.)
From the various facts now given, it is evident that most flowers are adapted in an admirable manner for cross-fertilisation. Nevertheless, the greater number likewise present structures which are manifestly adapted, though not in so striking a manner, for self-fertilisation. The chief of these is their hermaphrodite condition; that is, their including within the same corolla both the male and female reproductive organs. These often stand close together and are mature at the same time; so that pollen from the same flower cannot fail to be deposited at the proper period on the stigma. There are also various details of structure adapted for self-fertilisation. (10/19. Hermann Muller ‘Die Befruchtung’ etc. page 448.) Such structures are best shown in those curious cases discovered by Hermann Muller, in which a species exists under two forms,—one bearing conspicuous flowers fitted for cross-fertilisation, and the other smaller flowers fitted for self-fertilisation, with many parts in the latter slightly modified for this special purpose. (10/20. ‘Nature’ 1873 pages 44, 433.)