Darwinism (1889) Part 24

[Ill.u.s.tration: FIG. 28.

Malva sylvestris, adapted for insect-fertilisation.

Malva rotundifolia, adapted for self-fertilisation.]

Now these four methods are all apparently very simple, and easily produced by variation and selection. They are applicable to flowers of any shape, requiring only such size and colour as to attract insects, and some secretion of nectar to ensure their repeated visits, characters common to the great majority of flowers. All these methods are common, except perhaps the second; but there are many flowers in which the pollen from another plant is prepotent over the pollen from fertilisation, the same flower, and this has nearly the same effect as self-sterility if the flowers are frequently crossed by insects. We cannot help asking, therefore, why have other and much more elaborate methods been needed? And how have the more complex arrangements of so many flowers been brought about? Before attempting to answer these questions, and in order that the reader may appreciate the difficulty of the problem and the nature of the facts to be explained, it will be necessary to give a summary of the more elaborate modes of securing cross-fertilisation.

(1) We first have dimorphism and heteromorphism, the phenomena of which have been already sketched in our seventh chapter.

Here we have both a mechanical and a physiological modification, the stamens and pistil being variously modified in length and position, while the different stamens in the same flower have widely different degrees of fertility when applied to the same stigma,--a phenomenon which, if it were not so well established, would have appeared in the highest degree improbable. The most remarkable case is that of the three different forms of the loosestrife (Lythrum salicaria) here figured (Fig. 29 on next page).

(2) Some flowers have irritable stamens which, when their bases are touched by an insect, spring up and dust it with pollen. This occurs in our common berberry.

[Ill.u.s.tration: FIG. 29.--Lythrum salicaria (Purple loosestrife).]

(3) In others there are levers or processes by which the anthers are mechanically brought down on to the head or back of an insect entering the flower, in such a position as to be carried to the stigma of the next flower it visits. This may be well seen in many species of Salvia and Erica.

(4) In some there is a sticky secretion which, getting on to the proboscis of an insect, carries away the pollen, and applies it to the stigma of another flower. This occurs in our common milkwort (Polygala vulgaris).

(5) In papilionaceous plants there are many complex adjustments, such as the squeezing out of pollen from a receptacle on to an insect, as in Lotus corniculatus, or the sudden springing out and exploding of the anthers so as thoroughly to dust the insect, as in Medicago falcata, this occurring after the stigma has touched the insect and taken off some pollen from the last flower.

(6) Some flowers or spathes form closed boxes in which insects find themselves entrapped, and when they have fertilised the flower, the fringe of hairs opens and allows them to escape. This occurs in many species of Arum and Aristolochia.

(7) Still more remarkable are the traps in the flower of Asclepias which catch flies, b.u.t.terflies, and wasps by the legs, and the wonderfully complex arrangements of the orchids. One of these, our common Orchis pyramidalis, may be briefly described to show how varied and beautiful are the arrangements to secure cross-fertilisation. The broad trifid lip of the flower offers a support to the moth which is attracted by its sweet odour, and two ridges at the base guide the proboscis with certainty to the narrow entrance of the nectary. When the proboscis has reached the end of the spur, its basal portion depresses the little hinged rostellum that covers the saddle-shaped sticky glands to which the pollen ma.s.ses (pollinia) are attached. On the proboscis being withdrawn, the two pollinia stand erect and parallel, firmly attached to the proboscis. In this position, however, they would be useless, as they would miss the stigmatic surface of the next flower visited by the moth.

But as soon as the proboscis is withdrawn, the two pollen ma.s.ses begin to diverge till they are exactly as far apart as are the stigmas of the flower; and then commences a second movement which brings them down till they project straight forward nearly at right angles to their first position, so as exactly to hit against the stigmatic surfaces of the next flower visited on which they leave a portion of their pollen. The whole of these motions take about half a minute, and in that time the moth will usually have flown to another plant, and thus effect the most beneficial kind of cross-fertilisation.[145] This description will be better understood by referring to the ill.u.s.tration opposite, from Darwin's _Fertilisation of Orchids_(Fig. 30).

[Ill.u.s.tration: FIG. 30.--Orchis pyramidalis.]

_The Interpretation of these Facts._

Having thus briefly indicated the general character of the more complex adaptations for cross-fertilisation, the details of which are to be found in any of the numerous works on the subject,[146] we find ourselves confronted with the very puzzling question--Why were these innumerable highly complex adaptations produced, when the very same result may be effected--and often is effected--by extremely simple means? Supposing, as we must do, that all flowers were once of simple and regular forms, like a b.u.t.tercup or a rose, how did such irregular and often complicated flowers as the papilionaceous or pea family, the l.a.b.i.ates or sage family, and the infinitely varied and fantastic orchids ever come into existence? No cause has yet been suggested but the need of attracting insects to cross-fertilise them; yet the attractiveness of regular flowers with bright colours and an ample supply of nectar is equally great, and cross-fertilisation can be quite as effectively secured in these by any of the four simple methods already described.

Before attempting to suggest a possible solution of this difficult problem, we have yet to pa.s.s in review a large body of curious adaptations connected with insect fertilisation, and will first call attention to that portion of the phenomena which throw some light upon the special colours of flowers in their relation to the various kinds of insects which visit them. For these facts we are largely indebted to the exact and long-continued researches of Professor Hermann Muller.

_Summary of Additional Facts bearing on Insect Fertilisation._

1. That the size and colour of a flower are important factors in determining the visits of insects, is shown by the general fact of more insects visiting conspicuous than inconspicuous flowers. As a single instance, the handsome Geranium pal.u.s.tre was observed by Professor Muller to be visited by sixteen different species of insects, the equally showy G. pratense by thirteen species, while the smaller and much less conspicuous G. molle was visited by eight species, and G.

pusillum by only one. In many cases, however, a flower may be very attractive to only a few species of insects; and Professor Muller states, as the result of many years' a.s.siduous observation, that "a species of flower is the more visited by insects the more conspicuous it is."

2. Sweet odour is usually supplementary to the attraction of colour.

Thus it is rarely present in the largest and most gaudily coloured flowers which inhabit open places, such as poppies, paeonies, sunflowers, and many others; while it is often the accompaniment of inconspicuous flowers, as the mignonette; of such as grow in shady places, as the violet and primrose; and especially of white or yellowish flowers, as the white jasmine, clematis, stephanotis, etc.

3. White flowers are often fertilised by moths, and very frequently give out their scent only by night, as in our b.u.t.terfly-orchis (Habenaria chlorantha); and they sometimes open only at night, as do many of the evening primroses and other flowers. These flowers are often long tubed in accordance with the length of the moths' probosces, as in the genus Pancratium, our b.u.t.terfly orchis, white jasmine, and a host of others.

4. Bright red flowers are very attractive to b.u.t.terflies, and are sometimes specially adapted to be fertilised by them, as in many pinks (Dianthus deltoides, D. superbus, D. atrorubens), the corn-c.o.c.kle (Lychnis Githago), and many others. Blue flowers are especially attractive to bees and other hymenoptera (though they frequent flowers of all colours), no less than sixty-seven species of this order having been observed to visit the common "sheep's-bit" (Jasione montana). Dull yellow or brownish flowers, some of which smell like carrion, are attractive to flies, as the Arum and Aristolochia; while the dull purplish flowers of the Scrophularia are specially attractive to wasps.

5. Some flowers have neither scent nor nectar, and yet attract insects by sham nectaries! In the herb-paris (Paris quadrifolia) the ovary glistens as if moist, and flies alight on it and carry away pollen to another flower; while in gra.s.s of parna.s.sus (Parna.s.sia pal.u.s.tris) there are a number of small stalked yellow b.a.l.l.s near the base of the flower, which look like drops of honey but are really dry. In this case there is a little nectar lower down, but the special attraction is a sham; and as there are fresh broods of insects every year, it takes time for them to learn by experience, and thus enough are always deceived to effect cross-fertilisation.[147] This is a.n.a.logous to the case of the young birds, which have to learn by experience the insects that are inedible, as explained at page 253.

6. Many flowers change their colour as soon as fertilised; and this is beneficial, as it enables bees to avoid wasting time in visiting those blossoms which have been already fertilised and their nectar exhausted.

The common lungwort (Pulmonaria officinalis), is at first red, but later turns blue; and H. Muller observed bees visiting many red flowers in succession, but neglecting the blue. In South Brazil there is a species of Lantana, whose flowers are yellow the first day, orange the second, and purple the third; and Dr. Fritz Muller observed that many b.u.t.terflies visited the yellow flowers only, some both the yellow and the orange flowers, but none the purple.

7. Many flowers have markings which serve as guides to insects; in some cases a bright central eye, as in the borage and forget-me-not; or lines or spots converging to the centre, as in geraniums, pinks, and many others. This enables insects to go quickly and directly to the opening of the flower, and is equally important in aiding them to obtain a better supply of food, and to fertilise a larger number of flowers.

8. Flowers have been specially adapted to the kinds of insects that most abound where they grow. Thus the gentians of the lowlands are adapted to bees, those of the high alps to b.u.t.terflies only; and while most species of Rhinanthus (a genus to which our common "yellow rattle"

belongs) are bee-flowers, one high alpine species (R. alpinus) has been also adapted for fertilisation by b.u.t.terflies only. The reason of this is, that in the high alps b.u.t.terflies are immensely more plentiful than bees, and flowers adapted to be fertilised by bees can often have their nectar extracted by b.u.t.terflies without effecting cross-fertilisation.

It is, therefore, important to have a modification of structure which shall make b.u.t.terflies the fertilisers, and this in many cases has been done.[148]

9. Economy of time is very important both to the insects and the flowers, because the fine working days are comparatively few, and if no time is wasted the bees will get more honey, and in doing so will fertilise more flowers. Now, it has been ascertained by several observers that many insects, bees especially, keep to one kind of flower at a time, visiting hundreds of blossoms in succession, and pa.s.sing over other species that may be mixed with them. They thus acquire quickness in going at once to the nectar, and the change of colour in the flower, or incipient withering when fertilised, enables them to avoid those flowers that have already had their honey exhausted. It is probably to a.s.sist the insects in keeping to one flower at a time, which is of vital importance to the perpetuation of the species, that the flowers which bloom intermingled at the same season are usually very distinct both in form and colour. In the sandy districts of Surrey, in the early spring, the copses are gay with three flowers--the primrose, the wood-anemone, and the lesser celandine, forming a beautiful contrast, while at the same time the purple and the white dead-nettles abound on hedge banks. A little later, in the same copses, we have the blue wild hyacinth (Scilla nutans), the red campion (Lychnis dioica), the pure white great starwort (Stellaria Holosteum), and the yellow dead-nettle (Lamium Galeobdolon), all distinct and well-contrasted flowers. In damp meadows in summer we have the ragged robin (Lychnis Floscuculi), the spotted orchis (O.

maculata), and the yellow rattle (Rhinanthus Crista-galli); while in drier meadows we have cowslips, ox-eye daisies, and b.u.t.tercups, all very distinct both in form and colour. So in cornfields we have the scarlet poppies, the purple corn-c.o.c.kle, the yellow corn-marygold, and the blue cornflower; while on our moors the purple heath and the dwarf gorse make a gorgeous contrast. Thus the difference of colour which enables the insect to visit with rapidity and unerring aim a number of flowers of the same kind in succession, serves to adorn our meadows, banks, woods, and heaths with a charming variety of floral colour and form at each season of the year.[149]

_Fertilisation of Flowers by Birds._

In the temperate regions of the Northern Hemisphere, insects are the chief agents in cross-fertilisation when this is not effected by the wind; but in warmer regions, and in the Southern hemisphere, birds are found to take a considerable part in the operation, and have in many cases led to modifications in the form and colour of flowers. Each part of the globe has special groups of birds which are flower-haunters.

America has the humming-birds (Trochilidae), and the smaller group of the sugar-birds (Caerebidae). In the Eastern tropics the sun-birds (Nectarineidae) take the place of the humming-birds, and another small group, the flower-p.e.c.k.e.rs (Dicaeidae), a.s.sist them. In the Australian region there are also two flower-feeding groups, the Meliphagidae, or honey-suckers, and the brush-tongued lories (Trichoglossidae). Recent researches by American naturalists have shown that many flowers are fertilised by humming-birds, such as pa.s.sion-flowers, trumpet-flowers, fuchsias, and lobelias; while some, as the Salvia splendens of Mexico, are specially adapted to their visits. We may thus perhaps explain the number of very large tubular flowers in the tropics, such as the huge brugmansias and bignonias; while in the Andes and in Chile, where humming-birds are especially plentiful, we find great numbers of red tubular flowers, often of large size and apparently adapted to these little creatures. Such are the beautiful Lapageria and Philesia, the grand Pitcairneas, and the genera Fuchsia, Mitraria, Embothrium, Escallonia, Desfontainea, Eccremocarpus, and many Gesneraceae. Among the most extraordinary modifications of flower structure adapted to bird fertilisation are the species of Marcgravia, in which the pedicels and bracts of the terminal portion of a pendent bunch of flowers have been modified into pitchers which secrete nectar and attract insects, while birds feeding on the nectar, or insects, have the pollen of the overhanging flowers dusted on their backs, and, carrying it to other flowers, thus cross-fertilise them (see Ill.u.s.tration).

[Ill.u.s.tration: FIG. 31.--Humming-bird fertilising Marcgravia nepenthoides.]

In Australia and New Zealand the fine "glory peas" (Clianthus), the Sophora, Loranthus, many Epacrideae and Myrtaceae, and the large flowers of the New Zealand flax (Phormium tenax), are cross-fertilised by birds; while in Natal the fine trumpet-creeper (Tecoma capensis) is fertilised by Nectarineas.

The great extent to which insect and bird agency is necessary to flowers is well shown by the case of New Zealand. The entire country is comparatively poor in species of insects, especially in bees and b.u.t.terflies which are the chief flower fertilisers; yet according to the researches of local botanists no less than one-fourth of all the flowering plants are incapable of self-fertilisation, and, therefore, wholly dependent on insect or bird agency for the continuance of the species.

The facts as to the cross-fertilisation of flowers which have now been very briefly summarised, taken in connection with Darwin's experiments proving the increased vigour and fertility given by cross-fertilisation, seem amply to justify his aphorism that "Nature abhors self-fertilisation," and his more precise statement, that, "No plant is perpetually self-fertilised;" and this view has been upheld by Hildebrand, Delpino, and other botanists.[150]

_Self-Fertilisation of Flowers._

But all this time we have been only looking at one side of the question, for there exists an abundance of facts which seem to imply, just as surely, the utter uselessness of cross-fertilisation. Let us, then, see what these facts are before proceeding further.

1. An immense variety of plants are habitually self-fertilised, and their numbers probably far exceed those which are habitually cross-fertilised by insects. Almost all the very small or obscure flowered plants with hermaphrodite flowers are of this kind. Most of these, however, may be insect fertilised occasionally, and may, therefore, come under the rule that no species are perpetually self-fertilised.

2. There are many plants, however, in which special arrangements exist to secure self-fertilisation. Sometimes the corolla closes and brings the anthers and stigma into contact; in others the anthers cl.u.s.ter round the stigmas, both maturing together, as in many b.u.t.tercups, st.i.tchwort (Stellaria media), sandwort (Spergula), and some willow-herbs (Epilobium); or they arch over the pistil, as in Galium aparine and Alisma Plantago. The style is also modified to bring it into contact with the anthers, as in the dandelion, groundsel, and many other plants.[151] All these, however, may be occasionally cross-fertilised.

3. In other cases precautions are taken to prevent cross-fertilisation, as in the numerous cleistogamous or closed flowers. These occur in no less than fifty-five different genera, belonging to twenty-four natural orders, and in thirty-two of these genera the normal flowers are irregular, and have therefore been specially modified for insect fertilisation.[152] These flowers appear to be degradations of the normal flowers, and are closed up by various modifications of the petals or other parts, so that it is impossible for insects to reach the interior, yet they produce seed in abundance, and are often the chief means by which the species is continued. Thus, in our common dog-violet the perfect flowers rarely produce seed, while the rudimentary cleistogamic flowers do so in abundance. The sweet violet also produces abundance of seed from its cleistogamic flowers, and few from its perfect flowers; but in Liguria it produces only perfect flowers which seed abundantly. No case appears to be known of a plant which has cleistogamic flowers only, but a small rush (Juncus bufonius) is in this condition in some parts of Russia, while in other parts perfect flowers are also produced.[153] Our common henbit dead-nettle (Lamium amplexicaule) produces cleistogamic flowers, as do also some orchids.

The advantage gained by the plant is great economy of specialised material, since with very small flowers and very little expenditure of pollen an abundance of seed is produced.

4. A considerable number of plants which have evidently been specially modified for insect fertilisation have, by further modification, become quite self-fertile. This is the case with the garden-pea, and also with our beautiful bee-orchis, in which the pollen-ma.s.ses constantly fall on to the stigmas, and the flower, being thus self-fertilised, produces abundance of capsules and of seed. Yet in many of its close allies insect agency is absolutely required; but in one of these, the fly-orchis, comparatively very little seed is produced, and self-fertilisation would therefore be advantageous to it. When garden-peas were artificially cross-fertilised by Mr. Darwin, it seemed to do them no good, as the seeds from these crosses produced less vigorous plants than seed from those which were self-fertilised; a fact directly opposed to what usually occurs in cross-fertilised plants.

5. As opposed to the theory that there is any absolute need for cross-fertilisation, it has been urged by Mr. Henslow and others that many self-fertilised plants are exceptionally vigorous, such as groundsel, chickweed, sow-thistle, b.u.t.tercups, and other common weeds; while most plants of world-wide distribution are self-fertilised, and these have proved themselves to be best fitted to survive in the battle of life. More than fifty species of common British plants are very widely distributed, and all are habitually self-fertilised.[154] That self-fertilisation has some great advantage is shown by the fact that it is usually the species which have the smallest and least conspicuous flowers which have spread widely, while the large and showy flowered species of the same genera or families, which require insects to cross-fertilise them, have a much more limited distribution.

6. It is now believed by some botanists that many inconspicuous and imperfect flowers, including those that are wind-fertilised, such as plantains, nettles, sedges, and gra.s.ses, do not represent primitive or undeveloped forms, but are degradations from more perfect flowers which were once adapted to insect fertilisation. In almost every order we find some plants which have become thus reduced or degraded for wind or self-fertilisation, as Poterium and Sanguisorba among the Rosaceae; while this has certainly been the case in the cleistogamic flowers. In most of the above-mentioned plants there are distinct rudiments of petals or other floral organs, and as the chief use of these is to attract insects, they could hardly have existed in primitive flowers.[155] We know, moreover, that when the petals cease to be required for the attraction of insects, they rapidly diminish in size, lose their bright colour or almost wholly disappear.[156]

_Difficulties and Contradictions._

The very bare summary that has now been given of the main facts relating to the fertilisation of flowers, will have served to show the vast extent and complexity of the inquiry, and the extraordinary contradictions and difficulties which it presents. We have direct proof of the beneficial results of intercrossing in a great number of cases; we have an overwhelming ma.s.s of facts as to the varied and complex structure of flowers evidently adapted to secure this intercrossing by insect agency; yet we see many of the most vigorous plants which spread widely over the globe, with none of these adaptations, and evidently depending on self-fertilisation for their continued existence and success in the battle of life. Yet more extraordinary is it to find numerous cases in which the special arrangements for cross-fertilisation appear to have been a failure, since they have either been supplemented by special means for self-fertilisation, or have reverted back in various degrees to simpler forms in which self-fertilisation becomes the rule. There is also a further difficulty in the highly complex modes by which cross-fertilisation is often brought about; for we have seen that there are several very effective yet very simple modes of securing intercrossing, involving a minimum of change in the form and structure of the flower; and when we consider that the result attained with so much cost of structural modification is by no means an unmixed good, and is far less certain in securing the perpetuation of the species than is self-fertilisation, it is most puzzling to find such complex methods resorted to, sometimes to the extent of special precautions against the possibility of self-fertilisation ever taking place. Let us now see whether any light can be thrown on these various anomalies and contradictions.