Читать книгу The Romance of Plant Life - Elliot George Francis Scott - Страница 5

Man's ideas of the use of flowers – Sprengel's great discovery – Insects, not man, consulted – Pollen carried to set seed – Flowers and insects of the Whinstone Age – Coal Age flowers – Monkey-puzzle times – Chalk flowers – Wind-blown pollen – Extravagant expenditure of pollen in them – Flower of the pine – Exploding flowers – Brilliant alpines – Intense life in flowers – Colour contrasts – Lost bees – Evening flowers – Humming birds and sunbirds – Kangaroo – Floral clocks – Ages of flowers – How to get flowers all the year round – Ingenious contrivances – Yucca and fig – Horrible-smelling flowers – Artistic tastes of birds, insects, and man.

FOR many centuries flowers were considered as pleasing and attractive decorations stuck about the world in the same way as they are put in a drawing-room in order to give people pleasure. Very soon they were found to be extremely useful in poetry, sometimes to point a moral or disguise a sermon, like the primrose in Peter Bell, but more generally to produce a good impression on the BELOVED OBJECT. Burns puts the usual view of flowers very nicely in the following: "But I will down yon river rove amang the woods sae green, and a' to pu' a posie to my ain dear May." Possibly this is the meaning also in the exquisite lines of Shakespeare about the pansy: —

Even if there is no particular meaning, the "little western flower" gives point and beauty to the lines.

People only began to understand flowers about the year 1793, when Christian Conrad Sprengel, Rector of Spandau, near Berlin, published a very interesting work. He had discovered that the beauty of flowers and their colour and shape were by no means intended solely to please human eyes, but that they were designed to attract and allure the eyes of insects. Before his time there had been many guesses. Indeed, Theophrastus (born 371 B.C., and often mentioned in this work) seems to have quite well understood why flowers produce pollen, and that the fruit would not set and form seed unless pollen was carried to the female part of the flower. He mentions that the Pistacio has both male and female plants, and that Palms only form dates when the pollen is carried to the female tree. This experiment with the Date-palm was tried in 1592 by an Italian (Alpino) in an Egyptian tour, and the Englishman, Jacob Bobart, the Pole, Adam Zaluzianski (the latter in the same year) confirmed the general idea. Then in the year 1694 Rudolp Jacob Camerarius, a German, carried on a few more experiments, but no real definite advance was made until 1793, in the very midst of the French Revolution.25

The great point of Sprengel's discovery was in its being an intelligible explanation of the reason why flowers have bright colours, scent, and honey. At his time and indeed for many years afterwards, botanists looked on the stamens, petals, and other parts of the flower exactly in the way that a stamp collector looks at punctures and postmarks, that is without thinking about their meaning. Now we find that they are always designed to fulfil a perfectly definite purpose, and that all their details are contrived accordingly.

This purpose is to carry the pollen from the stamens of one flower to the stigma of another. The pollen can usually be recognized as a yellowish or reddish dust formed in the stamens; this dust is generally rubbed off on an insect's proboscis or on part of its body. When the insect reaches another flower the pollen is scraped off by a sticky or gummy stigmatic surface. When the pollen has been placed on this surface it grows, germinates, and part of it unites with the egg-cell of the young seed.

The latter is then, and not till then, able to become ripe and mature. It may be compared to cross-breeding in animals, though the process does not exactly correspond.

But all flowers do not require insects to carry their pollen. In early geological periods we do not find any flowers like those that now exist, nor in those early times were there any flies, bees, or butterflies.

The cockroach seems to have existed in Silurian (whinstone) times, and many gigantic and extraordinary insects lived in those damp forests of ferns, club-moss, and horsetails, of which the remains now form our British coalfields. Mayflies, plantbugs, and especially dragonflies (some of them with wings two feet across) existed, but none of these insects are of much use as pollen-carriers.

Even much later on, when screw pines, monkey-puzzle trees, ginkgos, and bamboos formed the forests and woods of Europe, crickets and earwigs existed; but it is not until that geological period in which the chalk was formed (the Cretaceous age) that fossil plants like most of those now familiar to us occur. These had flowers intended for insects, and with the fossil plants we find the fossils of the insects that visited them. Bees, butterflies, and ordinary flies appeared upon the scene just as soon as there were flowers ready for them. Mr. Scudder has even found the fossils of certain plants, and with them the fossils of butterflies closely allied to the present butterflies which now live on present trees allied to those fossils!

How then was the pollen of the first flowers carried?

It was in all probability blown by the wind or carried in water. Even now poplars, alders, birches, and oaks rely chiefly upon the wind to carry their pollen. These plants were amongst the first of our modern flora to appear upon the earth. Some of them possess very neat contrivances suited to the wind. The catkins of the alder, for example, hang downwards, so that each little male flower is protected from rain by a little scale or bract above it. The pollen is very light, dusty, or powdery, so as to fly a long distance. The Scotch fir (Pinus sylvestris) has male flowers in little cones. These are upright, and the pollen of each stamen drops on to a small hollow on the top of the stamen below. It is then blown away by the wind on a fine dry day, but it is not allowed to get out in wet weather. It is said that vast clouds of pine pollen occur in America, and that the water of certain lakes becomes quite yellow and discoloured by it at certain seasons. Each little particle of pollen has two minute caps or air-balloons which give it buoyancy, so that it can float easily immense distances.

A curious little herb, the Wall Pellitory, and another foreign species, the Artillery plant, produces small explosions of pollen. When it is touched, there is a little puff or cloud of dusty pollen. Even the common Nettle does the same on fine dry days when it is in full flower.

But of course this carrying of pollen by the wind is a very expensive arrangement. It is so much a matter of pure chance that a grain arrives at its right destination. Suppose that a flower is giving out clouds of pollen, then the chance of a pollen grain reaching a female flower only five feet away is very small, even if the stigma of the female flower is a quarter of an inch in diameter. The chance of pollen reaching it will only be about 1 to 1440; 1439 pollen grains will be wasted26 for every one that reaches the stigma. But even this is not quite a fair calculation, for if the female flower is not down wind, none will reach it at all!

But if an insect goes to the catkin of an alder or any other male flower, it will see the red points of the stigma and will very likely go there at once. This shows how much more reasonable and efficient insects will be.

The immense majority of flowers are, in fact, purple, blue, red, yellow, or white, so that they are conspicuous, and stand clearly out against the green of their leaves. It is well known to all who have arranged flowers for the table that the green of the leaves of different plants varies greatly in its shade and tint. Many greens do not match special flowers at all, but it is the fact that the green of any one plant is always quite harmonious, and agrees well with its own flowers!

Besides varied and beautiful colours, sweet or strong scents and supplies of honey or nectar are provided for insects.

How did flowers manage to produce all these attractions? No one has answered that question. We know in a general sort of way that the parts of flowers are modified leaves, and that petals and stamens become yellowish or pure white because they do not form green colouring matter like ordinary leaves.

It is also known that on the Alps or on any high mountain, where the air is pure and the sun strong, flowers become rich, brilliant, and vivid. In such places as the "Jardin" near Mont Blanc, the pure, deep, rich blue of gentians, the crimsons, reds, and purples of other flowers, impress the most casual and unobservant traveller. "White and red, yellow and blue, brown and green stand side by side on a hand's breadth of space." In that strong mountain air, also, perfumes are stronger, purer, and of finer quality than in the lowlands. There is a more intense, active, and vigorous life going on in flowers than is required by the more prosaic industries in other parts of a plant. Flowers also often live at a higher temperature than the surrounding air.

Kerner has described how the little flowers of Soldanella penetrate the snow by actually melting a passage for themselves through it (see p. 103).

This high temperature and vigorous life, shown also by the rapid transpiration of flowers,27 seems to hint that colours and perfumes appear in consequence of rapid chemical transformations.28

It was, of course, by degrees that the extraordinary variation in colour, which exists in nature, came about. No doubt bees, bumble-bees, wasps, and the more intelligent flies were improved and developed æsthetically. We can almost tell by looking at a flower what sort of insect probably visits it.

Not only so, but there are the neatest imaginable contrasts and blends of colour. The common Bluebeard Salvia, e.g., has the uppermost leaves (three-quarters to an inch long) of a deep, rich, blue-purple, which the roving Bumble-bee will see from a long way off. The Bumble-bee flies to this great splash of her favourite hue and for a second buzzes angrily, then she notes the small bright-blue patches on the upper lips of the small flowers below the leaves which are set off by white hairs of the upper and yellow hairs of the lower lip.

That bees really do understand and are guided by colour may be gathered from the following unfortunate accident. A certain hive of bees which had been brought up in a blue-striped skep became accidentally scattered. They tried to find their way back to their old home, but many strayed, and it was noticed that they had tried to enter the doors of every blue hive, which were strewn with the bodies of the unfortunate intruders.29

The rich blue-purple of Aconite, the dark strong red of the Woundwort (Stachys silvatica) are specially beloved by bumble-bees and hive-bees. Butterflies like any bright colour. Those flies which have a long, sucking proboscis, resemble the bees in their tastes, but all these insects are quite capable of finding out where they can get honey most easily, and visit flowers whatever the colour may be.

A very strange and wonderful fact is that quite a number of plants prefer the dark, or rather the dim, mysterious light of the gloaming. Then the Honeysuckle, the Evening Campion, the Night-scented Stock, Tobacco, and Schizopetalon give out their strongest scent and open out their white flowers as widely as possible. That is because they wish to attract the owlet moth and others which come out at this time, when there are fewer enemies and more security. If you look at any of these moth-flowers at mid-day, they are for the most part closed up, they are not particularly attractive, and they are giving out very little scent. The contrast to their condition in the evening is most striking.

Not only insects but birds are used to carry pollen. The gorgeous little humming birds, with their brilliant metallic crimson, bronze-green, and purple, are of the greatest importance in the New World. In the Old World they are replaced by the tiny Nectarinidæ or Sunbirds, with breastplates almost as exquisitely jewelled. They prefer the most gorgeous reds and scarlets, such as that of Salvia horminum, Lobelia cardinalis, and the like. Fuchsias are regularly visited by them in Tierra del Fuego, where sometimes they may be seen busily at work during a shower of snow. In South Africa they seize the stem of a Redhot Poker (Tritoma) (Kniphofia macowanii), and twisting their little heads round, they suck the honey from every blossom in succession. Still more interesting it is to see them perched on the edge of one of those great tumbler-like heads of Protea (e.g. P. incompta) and dipping their slender curved beaks repeatedly into the flowers. Then the little male bird will alight on a branch and make the most elaborate preparation for a song of triumph. Although helped out by fluttering of wings and much display of feathers and tail, the song is a very faint cheep of the feeblest description, and very difficult to hear.

Not only birds but even animals are sometimes called into the service. There is a group of small mammals which live on the honey of flowers. Even the Kangaroo is said to occasionally take a draught of nectar from some of the cup-like flowers of the Australian Dryandra (Proteaceae).

But one of the most interesting and extraordinary facts is the manner in which flowers fit in. They begin early in the morning: one blossom opens out and then another; all endeavouring to catch the attention of some passing insect. Allionia violacea opens at three or four a.m., and closes about eleven or twelve. Some wild Roses open about four or five in the morning, as well as the Chicory, Roemeria, etc. Virginian Spiderwort, Dandelion, and Nightshade are ready at six in the morning. A great many (Buttercups, White Water Lily, etc.) are open by seven a.m. Most of these early flowers are shut at noon. Others begin to close about three or four in the afternoon. The regular evening moth-flowers open about six p.m., though Cactus grandiflorus does not open till nine or ten p.m., and closes at midnight.30 Extraordinary as these variations seem, they are easily explained. Some open early because there are then few competitors. By far the greater number are open from nine a.m. till one or two p.m., because those hours are the favourite working time of most insects.

Flowers live for very different periods. That of the Wheat only lasts for fifteen or twenty minutes (its pollen is carried by wind), and is then over. There are others, Hibiscus and Calandrinia, which only remain open for three or four hours, but a Foxglove will last six days, a Cyclamen ten days, whilst Orchids may last for from thirty to eighty days (Cypripedium villosum, seventy days, Odontoglossum Rossii, eighty days).

Thus the sun every day through the summer, as he calls into life new swarms of insects, sees at every hour of the day new flowers opening their petals to his genial warmth and ready for the new bees and flies. The development of the flower and that of its insect are probably simultaneous, and equally regulated by the sun's warmth. Moreover the opening periods do not merely fit in during the day, but each flower has its own special month, and even in Scotland there is no month in which some flower may not be found in bloom. Any stray wandering insect can get its draught of honey at any season of the year.

This is a matter of some importance for those who keep bees, and the following list may be of some use. February: Crocus vernus, Snowdrop, Black Hellebore, and Hazel. March: The preceding, Arabis alpina, Bulbocodium, Cornus mascula, Helleborus fœtidus, Giant Coltsfoot, Gooseberry, various species of Prunus and Pyrus, Willow. April: The preceding as well as Adonis vernalis, Barbarea vulgaris, Brassica napus.

It is not worth while noting those that bloom from May to September, for there are hundreds of good bee-flowers in these months. In October: Borage, Echium, Sunflowers, Lycium europæum, Malope grandiflora, Catmint, Tobacco, Ocimum, Origanum, Phacelia tanacetifolia, and others. Most of these last into November.31 In December and January very few plants are in bloom. The following have been noted at Edinburgh Botanical Gardens: Dondia epipactis, Tussilago fragrans, Snowdrop, Geum aureum, Hepatica, Primula acaulis, P. veris, Aubrietia deltoidea, Crocus imperati, C. suaveolens, Erica herbacea alba, Helleborus (3 species), Polygala chamaebuxus, Andromeda floribunda; also Sir H. Maxwell32 mentions Azara integrifolia, Hamamelis arborea, and Chimonanthus fragrans. Of wild plants, Chickweed, Whin or Furze, Lamium purpureum, and Dandelion can generally be found in the depth of winter.

The contrivances which can be found in flowers, and by which the insect is forced to enter exactly along the proper path, are endless. Each flower has some little peculiarity of its own which can only be understood by thoroughly examining the plant itself. It is not therefore possible to do justice to the ingenuity of flowers in a work of this sort. There are orchids which throw their insect visitors into a bath of water, so that they have to crawl with wet wings up a certain path where they touch the pollen masses and stigma; others which hurl their pollen masses at the visitor. In the Asclepiads a groove is provided into which the leg of the insect slips, so that it has to struggle to get its foot out, and must carry off the pollen masses, though it often fails and leaves its leg behind. Some Arums and Aristolochias have large traps in which they imprison the insects, and only let them go when they are sure to be pollen-dusted. In one of these flowers there are transparent spots on the large petal-prison, which so attract the insects that they remain opposite them instead of flying out (just as flies do on a window-pane). Salvia has a stamen which is like a see-saw on a support; the bee has to lift up one end, which brings the other with its pollen flat down on to its back. The Barberry has a sensitive spot on its stamen; when the insect touches the spot, the stamen springs up suddenly and showers pollen upon it. In Mimulus the two flaps of the stigma close up as soon as they are touched, which will be when they have scraped off any pollen; then when the creature withdraws, covered with the flower's own pollen, none of this can be left on its own stigma, as this is shut up.

But instead of reading, one should watch a bumble-bee visiting the Foxglove flowers. The sight of her busily thrusting her great hairy body into the bell, which almost exactly fits her shape, while she gurgles with satisfaction, will teach the reader far more about the romance of flowers than many pages of description. If he then carefully examines the flower, he will see how the honey, the arched converging stamens, and the style, are placed exactly in the right place and where they will have the most effect.33

One orchid, Angraecum sesquipedale, has a spur eighteen inches long, and the great Darwin suggested that there must be an insect somewhere with a tube long enough to reach the honey. Such an insect, a large moth, was actually brought home from Madagascar, the place where this orchid occurs, after a lapse of many years!

Perhaps more remarkable than anything else are such cases as the Yucca and the Yucca-moth or the Fig-wasp and the Fig.

The Yucca is a fine lily-like plant resembling the Aloes in general appearance. A particular sort of moth lives entirely upon the Yucca. When the flowers open, the mother-moth kneads up a ball of pollen and places an egg inside. This ball she thrusts down the style into the ovary of the flower. There a grub develops from the egg and eats the pollen, yet some of this pollen fertilizes the young seeds. If Yuccas died out the moth would be exterminated. If the moths were destroyed, no Yuccas would ever set their seed!

The Fig has two sorts of flower. The one (caprifig) produces only male or pollen-yielding flowers. The other is the true edible fig. Inside the caprifig are the grubs of the fig-wasp, which rejoice in the name of Blastophaga grossorum. When grown up these force their way out of the caprifig and, flying to the true fig, the mother-wasp lays her eggs in certain flowers which have been apparently specially modified for the purpose. At the same time she covers the ordinary flowers with pollen from the caprifig. Her progeny return to the caprifig. Here again the future of a valuable fruit-tree is absolutely bound up with the fortunes of a tiny and in no way attractive wasp!

Another very remarkable case is that of those flowers (Stapelia, etc.), which in colour and general marking closely resemble decaying meat or other objectionable substances. Very often the smell of such flowers is exceedingly strong, and resembles the ordinary smell of putrid matter. In one case an artist employed to paint the flower had to use a glass bell, which was put over it. He could only lift it for a second or two at intervals in order to see the exact colour, before the horrible odour obliged him to cover it over again. Blowflies and others, which are in the habit of resorting to such substances, seek out these flowers in great numbers and lay their eggs upon them. In so doing they carry the pollen.

There are certain fungi which have quite as horrible a smell, and some of them also resemble decaying animal matter. These are most eagerly sought out by the same blow-and other flies (bright green lucilias, yellow-brown scatophagas, bluebottles, etc.). But in the case of these fungi it is the spores, not pollen, which is carried by the insect.

The effect of this flowery sort of life is abundantly evident in the structure of the insects themselves. Their mouth has been most wonderfully modified into a complex sucking apparatus; their legs have been transformed to act as pollen-carrying baskets, and the habits and tastes of the insects have been modified in the most extraordinary way.

Perhaps also the association of bright colours with a very pleasant sensation – that of a full, satisfying meal – has raised the artistic sensibilities of butterflies, sunbirds, humming birds, etc. For certainly these flower-haunting birds and butterflies are remarkable for their brilliant colouring. This has probably been brought about by the preference of the females for the most brilliantly coloured male butterflies and humming birds.

At any rate bright reds and blues are common to both bird or insect and to the flowers that they frequent. But the most curious point of this whole question lies in the fact that human beings of all grades, South Sea Islanders, the Ancient Greeks, Peruvians, Japanese, Romans, as well as the Parisians and Londoners of to-day, appreciate the beauty of colouring and grace of form which are so obvious in the world of flowers.

Yet man has had nothing whatever to do with the selection of either these colours or shapes. Many of those which he considers most precious (such as the weird, spotted, and outlandish Orchids of Madagascar and South America) have very likely scarcely ever been seen by man at all. It is to the artistic eye of the honey-bee, bumble-bee, butterfly, and of the humming bird and sunbird, that we owe these exquisite colours. The grace and beauty of outline probably depend upon their perfect symmetry and on the perfect suitability of every curve to its purpose.

Therefore it seems that the eyes of man, whether savage or civilized, are pleased and comforted by these same colours that delight the little brains of insects and birds.

This is indeed a mysterious fact.