Читать книгу: «The Romance of Plant Life», страница 19

The Imbauba tree (Cecropia spp.) also possesses a standing army of these ants. It puts them up in the hollow pith in the centre of the tree, which is divided into large roomy spaces and makes a convenient nest. There is a minute opening by which they run in and out. On one occasion a naturalist found that the ants had been benumbed by a period of very cold weather, and in consequence had neglected their duty, and the trees had been stripped of their leaves by leaf-cutting kinds.¹³²

These last mentioned, the leaf-cutting ants, are especially dreaded by owners of plantations. Foreign or introduced plants are not specially guarded against their ravages by special secretions, as is the case with the native flora, so that the coffee and cocoa plantations are often severely injured. In some places man has copied those Acacias and Imbaubas, for in the orange plantations of the province of Canton, in China, ants' nests are collected and placed on the trees. Moreover, the different trees are connected together by bamboos, so that the ants can easily pass, as on a bridge, from one tree to another.

Near Mantua, in Italy, the same system seems to be adopted, and ants' nests are carefully placed near the fruit trees. Their use can be quite well understood, for Forel, in his work on the Ants of Switzerland, estimates that one ants' nest will require a supply of 100,000 insects a day during the season.

It is quite common to find ants crawling about on the outside of the large heads of the Garden Centaury and a few other Composites. If one looks carefully, one finds that there are streaks of honey to be seen coming from the scales. The honey is not produced in the flowers, and seems at first sight to be of no use at all so far as the plant is concerned, but that is very far from being the case. Here comes a cockchafer or other destructive beetle, intent on absolutely devouring and destroying the young flowers. At once the pugnacity and wrath of the ants are aroused. They take up a menacing and ferocious attitude, and the cockchafer passes to some other plant.¹³³

Such honey-glands found on the leaves and not connected in any way with the flowers, are more common than one would think. Even the common Bracken produces curious honey-secreting hairs when it is in a young condition. These attract ants which drive away caterpillars and other dangerous insect foes.

Many very dangerous insects are too small for birds, and can only be dealt with effectually by insects or fungi. Of these perhaps the most dangerous are the "scale" insects. The best-known one is very like a minute mussel shell. It is about one-quarter to one-third of an inch long, and can be sometimes found in quantities on apples; they are generally collected round the stalk. The mother insect has this scaly back, and lies down and dies on the top of her eggs, so that her scaly corpse forms a roof and a shield for her young ones. Like all pests of this sort, these creatures increase very rapidly.

A certain scale insect was doing an immense amount of harm in the orange plantations of Fiji, but it was destroyed by the introduction of lady-birds, and of a certain parasitic fly. It is said that these insects destroyed the "scale" in six months!

Experiments have also been tried with fungi. There are certain fungi which attack the bodies of living insects. So far, however, it cannot be said that the results have been at all satisfactory, for the propagation and infection of the living insects by fungus spores is not at all easy. There is also a certain feeling of doubt as to what may happen. Those fungi, and particularly bacteria, might set up dangerous epidemics.

Decaying meal contains hundreds of certain very curious worms called Nematodes. They are short, about one-twenty-fifth of an inch in length, and are smooth and very like minute eels. These creatures are very active, wriggling or swaying to and fro in a characteristic manner. Now in decaying meal there is a peculiar fungus. Like most fungi, it consists of very minute transparent threads which contain living matter or protoplasm. This particular fungus has branches, but also forms curious loops or belts. When one of these eel-worms is swaying about in the meal, it may quite well happen that its tail slips into one of these loops. If that happens, the fate of the worm is sealed, for the loop is elastic, and the more it wriggles the farther it slips in and the stronger it is held. The fungus then begins to grow, and forms a tube which grows into the worm and kills it. All the material in the worm's body goes to nourish the fungus. This extraordinary fungus has been described and figured by Professor Zopf, but seems to be a very unusual and rare form.

CHAPTER XXIII
THE PERIL OF INSECTS

The Phylloxera – French sport – Life history of the Phylloxera – Cockchafer grubs – Wireworm – The misunderstood crows – Dangerous sucklings of greenflies – "Sweat of heaven" and "Saliva of the stars" – A parasite of a parasite of a parasite – Buds – The apple-blossom weevil – Apple-sucker – The codlin moth and the ripening apple – The pear midge – A careless naturalist and his present of rare eggs – Leaf-miners – Birds without a stain upon their characters – Birds and man – Moats – Dust and mites – The homes of the mites – Buds, insect eggs, and parent birds flourishing together.

THE difficulty in describing the Romance of Plant Life does not arise from a want of romance, but the sieges, battles, and alarms are so difficult to see, and the enemies are so tiny, that the terrific contests continually going on escape our notice altogether.

When one does look carefully and closely at the life of a plant, one sometimes wonders how it manages to exist at all in the midst of so many and great dangers.

There are great swarms of insects which devour or burrow into it, or suck its life-juices. These are infinitely more dangerous than the relatively clumsy, heavy-footed, grazing animal.

Every part of a plant has its own special insect foe, and it is really difficult to understand how it can possibly escape.

Perhaps the "Achilles' heel" is the root, for, underground, plants get no help from the watchful and ever-present army of birds, who are, as we shall see, the natural police of the world.

The Phylloxera, for instance, which ruined the old and valuable vineyards in France, is a terrible little acarid, or mite, which attacks the roots. Too small to see, and impossible to kill without killing the plant, it laid waste the fertile hills and valleys of all South and Central France, causing millions of pounds damage. One reason for this destruction sprang from the universal sporting instinct innate in every Frenchman. Everybody goes out with his gun to destroy any lark, sparrow, or titmouse that is idiotic enough to remain in the country. Only birds can deal efficiently with insect pests. Take this horrible little Phylloxera, for instance; a single female in her life of forty-five days will lay about two hundred eggs. Each egg becomes a little grub, which after a few moments of uncertainty and agitation settles itself, and begins to suck steadily at any unoccupied part of the vine root. After ten to twelve days' life it will be laying eggs as rapidly as its mother. Thus in an ordinary summer the number of young ones produced from a single female becomes quite incalculable.

These pests are natives of America. Imported on American roots about 1868, they had in thirteen years practically ruined the vineyards in France, Spain, Portugal, Italy, and Germany.

All sorts of remedies were tried – saturation of the ground by poisons, flooding the vineyards to drown them, artificial cultivation of their insect and plant enemies, and many others.

The correct and satisfactory method has been at last discovered. American vines of sorts which are able to resist these Yankee mites have been imported, and the valuable French vines have been grafted on to them.

Another very dangerous root-enemy, which is common in this country, is the Cockchafer grub or Whitegrub. (But it is not nearly so bad as in France, where in the summer of 1889, a single farmer collected 2000 lb. of Cockchafers.) The grub (each female lays seventy eggs) burrows into the earth, and for no less than three summers remains below ground devouring indiscriminately the roots of everything he can discover. Underground, the mole is almost his only enemy, but the rooks, starlings, and gulls, which follow the plough, are watching for him. The Wireworm, Clickbeetle, or Skipjack, is also an underground demon which lives for three years, and gnaws and worries at plant roots for the whole of that time. It, however, shows itself above the surface.

A gentleman who had passed his whole life in the country complained, in my presence, of the damage done by rooks. He had had six thousand of them shot that summer, and remarked that he had seen with his own eyes one of them pulling out a young cabbage plant by the root. Of course it was quite unnecessary to point out that the poor bird was merely trying to get at the wireworms and devour them!

For some time I used to look out for great attacks of wireworm in turnip-fields: when one was recorded, I never failed to find that the crows had been ruthlessly shot down a season or two before.

All these, and many other insects, attack the roots, which would be, one would suppose, quite well protected in the depths of the earth. Therefore we find roots producing all sorts of poisonous substances, tannins, and even strong-smelling bodies, which keep off these pests.

It is perhaps the sucking battalions of the insect army which do the most harm. In themselves they are weak, stupid, and scarcely move from their birthplace. They live out their life wherever their long, lancet-like proboscis needles have pierced the plant's skin, but it is their power of multiplication that makes them really formidable.

Huxley calculated that if all the offspring of one "green-fly" lived, and if their broods also lived for ten generations, then the tenth brood of that original green-fly would contain more animal matter than the entire population of China. Green-fly would, as a matter of fact, go on increasing at this rate, were it not for the enormous number of enemies that prey upon them. A mathematical friend of Mr. Buckton calculated that in 300 days the produce of a single green-fly might be 21015, that is 210 multiplied by 210, and then again by 210 up to 15 times!

In summer time one may often notice, especially on sycamores and lime trees, a peculiar shining, sticky, honey-like substance which covers the leaves. It is often so abundant as to drip like a rain of honey from the upper branches.

This "honey-dew" was a puzzle which greatly intrigued learned minds in the ancient world. Pliny speaks of it as the "sweat of heaven" or "saliva of the stars."

In reality, however, it is nothing but the excretions of hundreds of millions of these green-fly or aphides, which will be found established on the under side of the leaves, where, moored by their little anchoring talons and with their proboscis inserted in the fresh green leaf, they are sucking hard and steadily at the sugary juice. In twenty-four hours it was observed that a single individual gave forth forty-eight minute drops of honey.

Bees are very often tempted to collect this honey so abundantly produced, but this turns their own honey black, and may even make it poisonous.

Plants try to protect themselves against these pests chiefly by means of sticky or long hairs, by a thick skin, or by unpleasant tasting or smelling substances. But it is to insects such as lady-birds and others which devour the green-fly that they owe a deep debt of gratitude. In particular, there are certain parasitic insects which lay their eggs in their bodies. Not only so, but it is known that the eggs of some other insects are laid in the egg of the green-fly, and in one instance it has been found that yet another insect laid its egg in the egg of the parasite!

Some of the most interesting objects in nature are the buds in which, all neatly packed and stowed away, the young leaves and flowers remain awaiting the warm breath of spring. They are most interesting to examine: one finds series after series of overlapping scales which cover one another in the most ingenious way. No two are exactly alike, but each seems to have been moulded exactly to the proper shape. There is no waste anywhere, no useless expenditure of material. Very often turpentine or resin or a sticky gum seals up the joining of the scales. Every possible precaution seems to have been taken by nature. Neither rain nor snow can enter a winter bud. Neither can the cold of winter penetrate to the inside where the baby leaves and flower petals are cosily and tightly coiled up. But observe in the very earliest warm days of spring an extraordinary little insect, which has wakened up after its own winter sleep in the moss or lichen covering the rough and crannied bark of an old apple tree. This is the Apple-blossom Weevil, a beetle only about quarter of an inch in length, but with a curious snout or proboscis half the length of its body. This creature proceeds to the bud, and fixing its legs firmly, proceeds to bore a hole through the scales into the middle of the bud. She then places an egg inside, and goes on to put an egg in each of fourteen to forty-nine other buds. This takes a fortnight, and then she dies, probably satisfied that her duty is fully performed. A little footless, cream-white maggot develops in the apple-bud, which latter becomes rusty-coloured and dies away.

Another pest is the Apple-sucker, which lays her eggs in September on the fine hairs which cover the shoots. As soon as the weather becomes mild and warm, little grubs come out of these eggs; they are very small, and their bodies are almost flat. These tiny flat grubs, as soon as they are born, hurry off to the nearest buds and slip between their scales. They remain sucking the rich juices of the apple blossom until May or June, when they become perfect insects, and fly away so fat and well-nourished that they can live until September without feeding.

But those are by no means the only dangers. It is not till the apple blossom, which has escaped all those perils, opens in the spring time, after its petals have unfolded in the warm air and the young apple is already half formed, that the Codlin Moth begins to attack them. This tiny little moth is then extremely busy. She lays about fifty eggs, but only one on each young apple. It is put in the one weak spot of the apple, just at the top, in the base of the withered flower. The grub tunnels down to the core and feeds upon the seeds, which are entirely destroyed. When it has grown sufficiently, it drives another tunnel straight outwards to the skin. If the apple is still on the tree, the caterpillar lets itself down on a long silken thread and hurries off to hide in any convenient crack or crevice of the bark, or if the apple is already stored away, it conceals itself in the walls or in the flooring of the loft. The moths come out at the end of next May, just when the blossoms are getting ready for them. These codlin-moth apples cannot fail to have been noticed by the reader, as the tunnels in the ripe apple are most conspicuous. The gradual fattening of the caterpillar can also be traced, for its first tunnel down to the seeds is quite narrow, while the way out gets wider and wider as the creature became stouter and fatter whilst eating its way through the flesh.

The Pear Midge attacks at the same place, but the mother insect has a long egg-laying tube, and puts from fifteen to thirty eggs into the opening pear blossom. The pears go on growing, but of course are quite spoilt by the maggots within. These latter have a curious springing or jumping habit, and when they reach the soil bury themselves an inch or two below the surface.

So that all the care and neatness with which the young flowers and buds are packed up goes for nothing, and these insect pests get all the benefits of the apple and pear!

Besides these, there are hundreds of sorts of caterpillars which devour the leaves bodily. Cabbage-white butterflies, magpie-moths, gipsy-moths, diamondback-moths, and others, lay their eggs in hundreds. Many lay 300 eggs each.

In the United States, somebody had sent an entomologist a present of some eggs of one of these moths. They were placed on a paper near a window which happened to be open; the entomologist went out, and the paper must have blown across the street into a garden on the other side. At any rate, two or three years afterwards it was found that some trees were badly attacked by this moth. Nobody thought much about this, though of course it was interesting to find a new moth. But the pest became a very serious one. In consequence of the stimulating air of the United States the moth multiplied with the most extraordinary rapidity, and it is said that about 300,000 dollars was spent in one year in the attempt to stamp it out.

All this happened because an entomologist forgot to lock up his eggs when he went away for half an hour!

These caterpillars and the locusts devour the leaves bodily, but there are others which live inside them. These so-called "leaf-miner" caterpillars make white irregularly-winding tunnels between the upper and the lower skin of the leaf. The tunnel increases or widens because the caterpillar itself grows fatter as it eats its tunnel. They can be seen on a great many leaves, and can be at once recognized by this peculiarity.

Plants cannot run away from their enemies like animals, and it would seem at first sight that their case was very hopeless. But it is not so, for there is a vast, active, keen-eyed, and eager army of helpers always ready for eggs and caterpillars.

It is birds that are of the greatest importance. A titmouse will eat 200,000 insects in a season. A starling has been seen to fetch food for its young ones from a grass paddock 100 yards away no less than eighteen times in a quarter of an hour. All the following are excellent birds, and without a stain upon their characters: the plover, partridge, robin, wagtail, starling. Crows and wood-pigeons are under suspicion, for though the latter do good in devouring the seeds of weeds, and the former in destroying wireworms, both are fond of corn and take large quantities of it.

Thrushes, mavises, and blackbirds are amongst the most persevering and useful of our friends, but they are certainly fond of fruit. Yet the good which they do is very much more than any possible harm which an injudicious indulgence in the juicy fruits of summer might bring about.

The sparrow cannot be given a character. Indeed, he is objectionable in every way, for he not only does no good himself, but he devours corn and drives away starlings and other valuable and interesting helpers.

But it is very difficult to say what will happen if man interferes with the regular working of Nature. The starling has been a pest in Australia, though here it does nothing but good work. We are still grossly ignorant of many simple but very important facts. Even when we do know something, as for instance, that the peewit's or plover's whole life is occupied in clearing the ground of wireworm, daddy-long-legs grub, insects' eggs, and the like, that does not help the bird in the least. Plovers' eggs are regularly sold in enormous quantities. Every farm-labourer collects them, and the farmer never dreams of interfering.

Man shoots down owls, kestrels, hawks, who prey upon mice, voles, and sparrows. Then, when some farmers are half ruined, he has Royal Commissions to find out why the voles have increased so much.

There are one or two peculiar contrivances found in plants which are intended to keep off insects, and which may be noticed here.

Thus, the importance of a moat (which almost always formed part of the defence of a medieval castle) had been already found out by one or two plants.

In a particular kind of Teazle and in a large Sunflower-like Composite (Silphium laciniatum) every pair of two opposite leaves run together, so that a little cup-like hollow is formed surrounding the stem, in which water collects. Insects climbing up the stem and trying to get at the heads of flowers fall in and get drowned in this water; their bodies may be seen floating about in it, and probably when these decay, their decay-products are of some use to the plant.

This curious contrivance is only a development of a very common arrangement. In most leaves you will find that rainwater is intended to run in a particular direction. There are little grooves and canals down which it is supposed to go, and dry, thirsty hairs may be found so arranged as to intercept part of it. Thus in summer the plants are not confined entirely to the water from the ground, but are also refreshed by the rain from above.

But if you look closely along these little channels, and especially at the base of the leaf where they join the stem, you will find that dust particles washed down by the rain collect and form little streaks and patches. The air is full of all sorts of dust particles which are made up of every conceivable substance. Many of these minute grains of dust will be dissolved in the water, and help to supply the plant with food. Nor is that all, for if you take a hand-lens and examine these dust particles very closely, you will very probably find small animalcula moving about. They are not pretty; in fact they are quite horrible to look at. These are tiny mites which live in these places. Their office is probably to eat up everything eatable (including eggs of insects and spores of fungi), and their excreta as well as their own bodies will probably be dissolved in the water and go to help the plants.

The most certain place to find them is on the leaves of the lime and other trees in August. On the under side of the leaf little bushes of hairs can be found just where the veins fork. It is necessary to take a pin and stir up these hairs to frighten them out, but when this has been done, the lens will show the disgusting-looking little creatures running hurriedly away. They are no doubt exceedingly annoyed at being disturbed in the midst of their sleep, for they come out and forage for anything eatable at night, retiring for the day into these hairy grottos. The structure of these grottos is very complicated. They are often like little caves with a narrow entrance, and the sleeping chamber is quite within the leaf.

A great many trees have these curious mite homes. The insects are generally the colour of the hairs, and are not easy to distinguish.

All those insects mentioned here have so arranged their life histories that they come into existence exactly at the proper season. The warmth of the sun, which opens the apple buds ever so slightly, stirs also the egg of the mite, the egg of the beetle, or the hibernating weevil, so that all these insect populations come into full active life just when they can do the most damage.

But one must not stop there; the bird population is also ready, and is building its nests and feeding its young, just so soon as the insect swarms are at their thickest and most dangerous stage.

Man walks clumsily through this intricate tangle of living plants and animals: he sets his big foot on a hedgehog (good for the insects), or on a mole (so much the better for wireworm), collects plovers' eggs (to the great help of every insect), shoots an owl (to the delight of voles and mice) or a whole brood of partridges, and in other ways makes a – we had better say, shows that he is not so clever as he supposes himself to be.