Part 21

There are many other interesting cases of symbiosis between two different kinds of plants, and one side of the partnership is represented by fungi in a relatively large number of instances. The reason is not far to seek: fungi must always be dependent on other plants for their food; they must be parasitic, because they cannot themselves produce the organic substances they require. They must therefore associate themselves in some way with other organisms, living or dead, and as a general rule they simply prey upon their associate, sucking up its juices and killing it. But in not a few cases they can render services in return, and, as we have seen in the case of the lichens, symbiosis may then occur. Fungi in general have the power of discovering and absorbing the least trace of water in the soil, and with it they absorb the salts necessary to the plant, and in this, apparently, consists the service which they are able to render even to large plants fixed deep in the earth, such as shrubs and trees. The roots of many of our forest trees, e.g. beech, oak, fir, silver poplar, and bushes like broom, heaths, and rhododendrons, are thickly wrapped round with a network of fungoid threads, and the mutual relations just indicated exist between these and the plants in question (Fig. 39, _A_ and _B_). The plants give to the fungi some contribution from the superfluity of their food-stuffs, and receive in return water and salts, which are of value especially in times of drought. Perhaps there is some connexion between this and the fact that limes wither and lose their leaves so quickly during great summer-heat; these and many other of our trees possess no root-fungi or mycorhizæ.

It is easy to understand, therefore, that genuine 'symbiosis' may have arisen from parasitism. But that this is not the only path that leads to symbiosis is shown by the cases of animal symbiosis we have already discussed.

The partnership between polyps and hermit-crabs may have arisen from a one-sided commensalism, since polyps establishing themselves on mollusc shells which were often made use of by hermit-crabs would be better fed than those which settled down on stones. There are still species which make use of both modes of settlement. Then followed the adaptation of the crustacean to the polyp, for, first, those hermit-crabs would thrive best which tolerated the presence of the polyp; then those which sought its presence, that is to say, which gave a preference to shells covered with polyps; and, finally, those which would take no others, and even themselves fixed the sea-anemone upon it, if it chanced to be removed. Intelligence need not be taken into account in the matter at all, not even in the hermit-crab's case. We have only to recall the complex instincts, exercised only once in a lifetime, which compel the silkworm and the emperor moth to elaborate their effective cocoons. The elaboration of the spinning-instinct can only be due to natural selection, for the insect can have had no idea of the utility of its performance, and the same is true in the case of the sea-anemones or the hydroid polyps and the hermit-crab. The sea-anemone is quite unconscious that it is defending its partner, the hermit-crab, when it lashes out its stinging acontia on any disturbance, and the hermit-crab is equally unaware that the sea-anemone is contributing to its safety; both animals act quite unconsciously, purely instinctively, and the origin of these instincts, on which the symbiosis is based, must be due, not to intelligent activities which have become habitual, but only to the survival of the fittest.

According to the principle of natural selection nothing can arise but that which is of use directly or indirectly to its possessor. Nevertheless, there are cases in which it appears as if something had arisen, which was of no use to the species in which the variation appeared, but only to the species protected by it. This is the case in the remarkable symbiosis between algæ of the family Nostocaceæ and the floating, moss-like water-fern _Azolla_. This plant, in external appearance almost like duckweed, has on the under surface of its leaves a minute opening, leading into a relatively roomy hair-lined cavity, and in this cavity there is always, enclosed in jelly, a bluish green unicellular alga, _Anabæna_. The cavity is present in every leaf, and the alga is present in every cavity, making its way in from a deposit of alga-cells which is found on the incurved tip of every young shoot. As soon as a young leaf of _Azolla_ unfolds from the bud it receives its _Anabæna_ cells from this deposit, and no one has yet found either twigs or leaves which were free from the algæ. But no one has succeeded in discovering any benefit derived by the _Azolla_ from this partnership.

This looks like a contradiction of the theory of selection, but there remains the possibility that there is some benefit rendered to the _Azolla_ by the alga, though we cannot see it as yet. There is also the possibility that the cavity is an organ which was of use to the plant at an earlier time, perhaps as an insect-trap, but has now lost its significance, and is utilized by the alga as a dwelling-place. This, however, is contradicted by the remarkable distribution of the four known species of _Azolla_. Two of these are widely distributed in America; the third lives in Australia, Asia, and Africa; the fourth in the region of the Nile: all four have cavities in their leaves, and in all these forms the cavity is inhabited by the same species of _Anabæna_. This indicates that the leaf-cavity and the partnership with the alga must have originated in remote antiquity; the symbiosis must date from a time before the four modern species of _Azolla_ had split off from a single parent-species. But no rudimentary organ, that is to say, no organ not of use to the plant itself, would have been preserved through such a vast period of time, as we shall see later, for useless organs disappear in the course of ages. As the cavity has not yet disappeared, we may assume with some probability that it is useful to the plant, whether by means of the _Anabæna_, or in some other unknown way. To draw an argument against the reality of the processes of selection from our lack of knowledge of what this advantage may be would be as unreasonable as if, notwithstanding our experience that stones sink in the water, we were to assume of a particular stone which we did not see sink, because it was hidden from our sight by bushes, that perhaps it had not sunk, but was capable of floating.

LECTURE X

THE ORIGIN OF FLOWERS

Introduction--Precursors of Darwin--Pollination by wind--Arrangements in flowers for securing cross-fertilization--Salvia, Pedicularis--Flowers visited by flies--Aristolochia--Pinguicula--Daphne--Orchids--Flowers are built up of adaptations--Mouth-parts of insects--Proboscis of butterflies--Mouth-parts of the cockroach--Of the bee--Pollen baskets of bees--Origin of flowers--Attraction of insects by colour--Limitation of the area visited--Nägeli's objection to the theory of selection--Other interpretations excluded--_Viola calcarata_--Only those changes which are useful to their possessors have persisted--Deceptive flowers--Cypripedium--Pollinia of Orchis--The case of the Yucca-moth--The relative imperfection of the adaptations tells in favour of their origin through natural selection--Honey thieves.

WHEN one species is so intimately bound up with another that neither can live for any length of time except in partnership, that is certainly an example of far-reaching mutual adaptation, but there are innumerable cases of mutual adaptation, in which, although there is no common life in the same place, yet the first form of life is adjusted in relation to the peculiarities of the second, and the second to those of the first. One of the most beautiful, and, in regard to natural selection, the most instructive of these cases is illustrated by the relations between insects and the higher plants, relations which have grown out of the fact that many insects have formed the habit of visiting the flowers of the plants for the sake of the pollen. In this connexion the theory of selection has made the most unexpected and highly interesting disclosures, for it has informed us how the flowers have arisen.

In earlier times the beauty, the splendour of colour, and the fragrance of flowers were regarded as phenomena created for the delight of mankind, or as an outcome of the infinite creative power of Mother Nature, who loves to run riot in form and colour. Without allowing our pleasure in all this manifold beauty to be spoilt, we must nowadays form quite a different conception of the way in which the flowers have been called into being. Although here, as everywhere else in Nature, we cannot go back to ultimate causes, yet we can show, on very satisfactory evidence, that the flowers illustrate the reaction of the plants to the visits of insects, and that they have been in large measure evoked by these visits. There might, indeed, have been blossoms, but there would have been no flowers--that is to say, blossoms with large, coloured, outer parts, with fragrance, and with nectar inside, unless the blossoms had been sought out by insects during the long ages. Flowers are adaptations of the higher flowering plants to the visits of insects. There can be no doubt about that now, for--thanks to the numerous and very detailed studies of a small number of prominent workers--we need not only suppose it, we can prove it with all the certainty that can be desired. The mutual adaptations of insects and flowers afford one of the clearest examples of the mode of operation and the power of natural selection, and the case cannot therefore be omitted from lectures on the theory of descent.

That bees and many other insects visit flowers for the sake of the nectar and pollen has been known to men from very early times. But this fact by itself would only explain why adaptations to flower-visiting have taken place in these insects to enable them, for instance, to reach the nectar out of deep corolla-tubes, or to load themselves with a great quantity of pollen, and to carry it to their hives, as happens in the case of the bees. But what causes the plants to produce nectar, and offer it to the insects, since it is of no use to themselves? And further, what induces them to make the pillage easier to the insects, by making their blossoms visible from afar through their brilliant colours, or by sending forth a stream of fragrance that, even during the night, guides their visitors towards them?

As far back as the end of the eighteenth century a thoughtful and clear-sighted Berlin naturalist, Christian Konrad Sprengel, took a great step towards answering this question. In the year 1793 he published a paper entitled 'The Newly Discovered Secret of Nature in the Structure and Fertilization of Flowers[8],' in which he quite correctly recognized and interpreted a great many of the remarkable adaptations of flowers to the visits of insects. Unfortunately, the value of these discoveries was not appreciated in Sprengel's own time, and his work had to wait more than half a century for recognition.

[8] _Das neu-entdeckte Geheimniss der Natur im Bau u. der Befruchtung der Blumen_, Berlin, 1793.

Sprengel was completely dominated by the idea of an all-wise Creator, who 'has not created even a single hair without intention,' and, guided by this idea, he endeavoured to penetrate into the significance of many little details in the structure of flowers. Thus he recognized that the hairs which cover the lower surface of the petals of the wood-cranesbill (_Geranium sylvaticum_) protect the nectar of the flower from being diluted with rain, and he drew the conclusion, correct enough, though far removed from our modern ideas as regards the directly efficient cause, that the nectar was there for the insects.

He was also impressed by the fact that the sky-blue corolla of the forget-me-not (_Myosotis palustris_) has a beautiful yellow ring round the entrance to the corolla-tube, and he interpreted this as a means by which insects were shown the way to the nectar which is concealed in the depths of the tube.

We now know that such 'honey-guides' are present in most of the flowers visited by insects, in the form of spots, lines, or other marking, usually of conspicuous colour, that is, of a colour contrasting with the ground colour of the flower. Thus, in species of Iris, regular paths of short hairs lead the way to the place where the nectar lies. In the spring potentilla (_Potentilla verna_) (Fig. 40) the yellow petals (_A_, _Bl_) become bright orange-red towards their bases, and this shows the way to the nectaries, which lie at the bases of the stamens (_st_), and are protected by hairs, the so-called 'nectar-covers' (_Saftdecke_) of Sprengel, from being washed by rain.

The recognition of the honey-guides led Sprengel on to the idea that the general colouring of the flower effects on a large scale what the honey-guides do in a more detailed way--it attracts the attention of passing insects to where nectar is to be found; indeed, he went an important step further by recognizing that there are flowers which cannot fertilize themselves, in which the insect, in its search for honey, covers itself with pollen, which is then rubbed off on the stigma of the next flower visited, fertilization being thus effected. He demonstrated this not only for the Iris, but for many other flowers, and he drew the conclusion that 'Nature does not seem to have wished that any flower should be fertilized by its own pollen.' How near Sprengel was to reaching a complete solution of the problem is now plain to us, for he even discovered that many flowers, such as _Hemerocallis fulva_, remained infertile if they were dusted with their own pollen.

Even the numerous experiments of that admirable German botanist, C. F. Gärtner, although they advanced matters further, did not suffice to make the relations between insects and flowers thoroughly clear; for this the basis of the theory of Descent and Selection was necessary. Here, again, it was reserved for Charles Darwin to lead the way where both contemporaries and predecessors had been blindly groping. He recognized that, _in general_, self-fertilization is disadvantageous to plants; that they produce fewer seeds, and that these produce feebler plants, than when they are cross-fertilized; that, therefore, those flowers which are arranged to secure cross-fertilization have an advantage over those which are self-fertilized. In many species, as Sprengel had already pointed out, self-fertilization leads to actual infertility; only a few plants are as fertile with their own pollen as with that of another plant; and Darwin believed that, in all flowering plants, crossing with others of the same kind, at least from time to time, is necessary if they are not to degenerate.

Thus the advantage which the flowers derive from the visits of insects lies in the fact that insects are instrumental in the cross-fertilization of the flowers, and we can now understand how the plant was able to vary in a manner favourable to the insect-visits, and to exhibit adaptations which serve exclusively to make these visits easier; we understand how it was possible that there should develop among flowers an endless number of contrivances which served solely to attract insects, and even how, for the same end, the insignificant blossoms of the oldest Phanerogams must have been transformed into real flowers.

We must not imagine, however, that the obviously important crossing of plant-individuals, usually called 'cross-pollination,' can be effected only by means of insects. There were numerous plants in earlier times, and there is still a whole series in which cross-fertilization is effected through the air by the wind; these are the anemophilous or wind-pollinated Angiosperms.

To these belong most of the catkin-bearers, such as hazel and birch, and also the grasses and sedges, the hemp and the hop, and so forth. In these plants there is no real flower, but only an inconspicuous blossom, without brightly-coloured outer envelopes, without fragrance or nectar; all of them have smooth pollen grains, which easily separate into fine dust and are carried away by the wind until they fall, by chance, far from their place of origin, on the stigma of a female blossom.

By far the greater number of the phanerogams, however, especially all our indigenous 'flowers,' are, as a rule, fertilized by means of insects, and it is amazing to see in what diverse ways, often highly specialized, they have adapted themselves to the visits of insects. Thus there are flowers in which the nectar lies open to view, and these can be feasted on by all manner of insects; there are others in which the nectar is rather more concealed, but still easily found, and reached by insects with short mouth-parts, e.g. large flowers blooming by day and bearing much pollen, like the Magnolias. These have been called beetle-flowers, because they are visited especially by the honey-loving Longicorns.

Other flowers blooming by day are especially adapted to fertilization by means of bees; they are always beautifully coloured, often blue; they are fragrant, and contain nectar deep down in the flower, where it can only be reached by the comparatively long proboscis of the bee. Different arrangements in the different flowers secure that the bee cannot enjoy the nectar without at the same time effecting the cross-pollination. Thus the stamens of the meadow sage (_Salvia pratensis_) are at first hidden within the helmet-shaped upper lip of the flower (Fig. 41, _st´_), but bear lower down on their stalk a short handle-like process, which turns the pollen-bearing anther downwards (_st´´_) as soon as it is pressed back by an intruding insect. The pollen-sacs then strike downwards on the back of the bee, and cover it with pollen. When the bee visits another more mature flower, the long style, which was at first hidden within the helmet, has bent downwards (_gr´´_), and now stands just in front of the entrance to the flower, so that the bee must rub off a part of the pollen covering its back on to the stigma, and fertilization is thus effected.

There are other flowers which are specially disposed to suit the visits of the humble-bees, as, for instance, _Pedicularis asplenifolia_, the fern-leaved louse-wort, a plant of the high Alps (Fig. 42). The first thing that strikes us about this plant is the thickly tufted hair covering on the calyx (_k_), which serves to keep off little wingless insects from the flower; then there is the strange left-sided twisting of the individual flowers, whose under lip allows only a strong insect like the humble-bee to gain access, towards the left, to the corolla-tube (_kr_), in the depths of which the nectar is concealed. While the humble-bee is sucking up the nectar it becomes dusted over with pollen from the anthers, which falls to dust at a touch, and when it insinuates itself into a second flower its powdered back comes first into contact with the stigma of the pistil (_gr_) which projects from the elongated bill-shaped under lip, dusting it over with the pollen of the first visited flower. Butterflies and smaller bees cannot rob this flower; it is strictly a humble-bee's flower.

There are not a few of such flowers adapted to a very restricted circle of visitors, and in all of them we find contrivances which close the entrance to all except what we may call the welcome insects; sometimes there are cushions of bristles which prevent little insects from creeping up from below, or it is the oblique position of the flower which prevents their getting in from the stem; sometimes it is the length and narrowness of the corolla-tube, or the deep and hidden situation of the nectar, which only allows intelligent insects to find the treasure.

Very remarkable are those flowers which are adapted to the visits of flies, for they correspond in several respects to the peculiarities of these insects. In the first place, flies are fond of decaying substances and the odours given off by these, and so the flowers which depend for their cross-fertilization on flies have taken on the dull and ugly colours of decay, and give out a disagreeable smell. But flies are also shy and restless, turning now hither, now thither, and cannot be reckoned among the 'constant' insect visitors, that is to say, they do not persistently visit the same species; it is, therefore, evident that they might easily carry away the pollen without any useful result ensuing. Moreover, their intelligence is of a low order, and they do not seek nectar with the perseverance shown by bees and humble-bees. It is not surprising, therefore, to find that many of the flowers adapted for the visits of flies are so constructed that they detain their visitors until they have done their duty, that is to say, until they have effected, or at least begun, the process of cross-pollination.