CHAPTER V
ALLIES OF THE PLANT WORLD
“I wish I were a willow tree—
Young wind in the green hair of me
And old brown water round my feet,
And a familiar bird to greet.”
—Elizabeth Fahnestock.
Every division of terrestrial life constitutes a struggle. The plants grow and carry on their business and social activities so unobtrusively that we seldom think of them as appealing to arms—yet their whole existence is a battle royal. They must fight with aspiring neighbours for every inch of their upward growth, and at the same time wage incessant warfare against a hundred insects and animal foes.
Under such strenuous conditions, it is only to be expected that the plants should seek profitable alliances with birds, insects and animals having interests similiar to their own. Such pacts are described by botanists as examples of symbiosis; they most frequently occur between plants and insects, but the plants also have their working agreements with members of the other two great kingdoms of life. In fact, all Nature is a vast system of checks and balances, with every creature preying more or less upon every other creature, except when they can gain more by joining their efforts. Certain Humming-Birds lie in wait near plants which by their nectar-sweets attract swarms of insects, and hard by, Snakes lie in wait for the Birds. The Birds rid the plants of destroying pests; the part of the Snakes in a beneficent scheme of existence is not so apparent, but merely because we cannot see good in a thing is no argument that it does not exist.
Many of the most important alliances of plants are made in response to the law that “Nature abhors perpetual self-fertilization”. This principle is one of the greatest in plantdom; there is a constant necessity for the intercrossing of independent life-streams. The plants go to great lengths to see that the multiplication and evolution of the species is properly carried on.
We always associate Bees and flowers, yet it is probable, that, as a whole, the plants, especially in the tropics, depend more upon Ants than upon any other insects. Many vegetable folk deliberately employ them to keep their leaves and stalks free of obnoxious visitors. The Cow-Horn Orchid, like most plants which perch on trunks and branches, produces pseudo-bulbs into which its vitality can recede in dry seasons. There is always a small opening at the bottom of each of these little tubes, through which Ants enter. They honeycomb the interior with cells and galleries where they can be perfectly dry in the wettest weather. On the approach of Caterpillars, Cockroaches and other Orchid enemies, the residents issue in great swarms to protect their combined host and home.
The species Coryanthes, instead of pseudo-bulbs, grows great masses of fibrous aerial roots among which the Ants dwell. They are ever ready to repel invasions of Cockroaches and other crawlers who seek to eat the tender growing root-tips.
An Epiphyte which is particularly solicitous for the welfare of its insect allies is the Ant-nest Plant, Rubiaceae Myrme. This ingenious creature not only builds nests but builds them made-to-order. Certain enlargements on its stem are hollowed out into chambers with connecting galleries quite ready for their intended tenants. All the Ants have to do is to move in. The kind that usually enter the plant’s service are fierce warriors, Iridiomyrmex Myrmecodiae, with very powerful stings. They form a formidable bodyguard.
Sometimes the Ant warriors of such compacts are quite satisfied to accept the free rental of their snug quarters as sufficient pay and seek their food elsewhere. More frequently, the alliance includes “board and lodging” with the plant issuing wages in the form of nectar, sweet pulp and other food.
The Cherry and Vetch are among plants which secrete a candy-like substance on their stalks which serves as an allurement for Ants to climb and establish their homes there. In many cases, these excretions are also barriers which prevent the Ants from hunting among the plant’s blossoms for honey, as they would thus destroy the precious grains of pollen.
The South American Imba-uba Tree, Cecropia, has a hollow trunk in which Bees and Ants dwell together amicably. The Polygonums Tree of the same continent has so many Ant allies that it is often entirely hollowed out by them. The process often operates so far that men break off the smaller twigs and use them as ready-made pipe stems. The Melastroma Plant of South America provides pouches on each leaf-stalk for the benefit of its black guardian Ants. The Tococas and Mermidones also have Ant-sacs.
In China it is a common practice of the Orange-growers to encourage the visitation of non-vegetarian Ants by placing selected species on trees and connecting the trees by bamboo poles over which the faithful insects can rush their forces to particularly threatened points.
Everyone knows of the large part the industrious Bee plays in the economy of the plant world. Few plants, there are, which are not aided in their love-making by this tiny brown buzzer; some flowers depend upon him entirely in their efforts to propagate the species.
The Bees and their relatives are particularly welcome to the flowers because they do the work of fertilization so well. Wingless insects are undesirable because they offer little guarantee that they will successfully carry pollen to some other flower of the same species. Even if it is not brushed off in the course of their laborious travels, they are not at all particular what kind of flowers they visit and so offer small hope of carrying pollen to its correct destination. Flying insects of the Bee family seem to have the work of cross-fertilization directly assigned to them. On each of their separate, pollen-gathering journeys, they are partial to one particular kind of flower. As they flit from blossom to blossom of the same species, going in and out of flower and flower, rubbing against a group of stamens here and brushing against a pistil there, they fertilize plant after plant in grateful acknowledgment of the store of sweets they are collecting.
Many and ingenious are the methods which flowers adopt to make sure that only invited and useful guests come to their nectar-feasts. The very Ants which guard the lower portions of a plant so well, might become mere greedy plunderers, if allowed to crawl within the flowers. It is not often that they do. Sometimes, the stalks and even the petals of flowers like the Rock-Lichens and the Butter-Wort are coated with some plant chemical exceedingly disagreeable for an insect to crawl over. Various alkaloids, resins and oils in the cell juices also make the flower and its leaves obnoxious to grazing animals. Many plants, like the Mullein and Stinging-Nettle, use bristles and prickles to repel Slugs and Caterpillars.
A common protective device is for a flower to place its nectar at the bottom of a long, narrow tube only accessible to a flying insect having a proboscis. In the Antirrhinum the entrance to the flower is closed to small crawlers by a very heavy corolla. Bees, because of their size and strength, can force their way through. It is said that as soon as the stigma of this flower has been fertilized, the corolla relaxes and Ants and their kind are free to enter and partake of such dainties as are left.
Nettles, Passion-flowers, and Lilies frequently line their interiors with stiff, in-pointing hairs which oppose a most effective palisade against anything that crawls, whereas a flyer provided with a proboscis can stand on the edge and, inserting his straw, drink up the best soda water in plantdom. This existence of proboscides in insects which help to cross-fertilize flowers is the very finest example we have of true mutualism. Here is a case where members of two supposedly different worlds of life have developed highly specialized organs in order that they might help each other.
It is said that Charles Darwin, after noting the extraordinary length of the spur of the Orchid Angraecum Sesquipedale of Madagascar predicted that some day there would be found in that country a moth with a proboscis ten to eleven inches long. Not many years after, Dr. Fritz Müller verified the sagacity of the famous scientist by finding an insect exactly answering this description.
The Birth-Wort (Aristolochia Clematitis) takes no chances with its insect visitors. In entering it, a Bee brushes easily by the down-pointing hairs only to find that, when he attempts to go out again, the bristles present stiff, unyielding obstacles against his egress. In his excitement at this discovery, he buzzes around quite angrily and, without noticing it, thoroughly showers the stigma with pollen and incidentally covers his own body with a good supply to be carried on to the next stop. When this process is quite complete, the flower graciously relents, relaxes its hairs and allows the exasperated insect to escape.
The Pedicularis family uses similiar coercive methods, and by sharp teeth, forces insect-visitors to take a course through the flowers which brings them in contact with both stamens and pistils.
The purple Loosestrife, pretty dweller by banks and meadows, sets a rich table and so always has plenty of insect visitors. It produces six different kinds of yellow and green pollen, and is therefore sure to suit every taste. Incidentally it has two different sets of stamens and stigmas of three different lengths.
Night-blooming flowers only entertain after the sun goes down. All day long they look withered and dead, but with the coming of the stars, they open up to show conspicuous white or light-tinted interiors. A flower like the Silene also exhales a rich, sensuous odor, which, with its light colour, serves to attract such insects as are abroad at night.
Sycamore and Lime trees have humble allies in the tiny mites which live in the retreats built of hairs to be found at the places where the veins of the leaves fork. During the day they hide away from sight, but at night they come out and scour the leaves clean of noxious bacteria and fungus spores.
Pollen of different plants, when examined under the microscope, reveals wonderful facts about the reciprocal relations which exist between plants and insects. Wind-fertilized plants are nearly always without any special beauty of form, colour or scent, while plants which are fertilized by insects are most always conspicuous, brightly coloured and highly scented. In the same way, pollen of the Hazel, Birch, and Balsam Poplar, which is carried by the wind, is small, light, practically spherical and devoid of protuberances. Pollen of the Primrose, Cowslip and Polyanthus, often carried by insects, is deeply furrowed, covered with spines and knobs, strung together by sticky threads and, in other ways, provided with apparatus which enables it to adhere to any object which it touches.
The pollen of the Hollyhock and the Dandelion consists of large, beautiful, spherical grains covered with spikes. The Rhododendrons, Azalias, and Fuchsias produce great masses of grains bound together by viscid threads. Many of these bits of life-principle are geometric masterpieces. A pollen grain of the Cobaea Scandens is one of the most fascinating objects of the microscopic world. It is perfectly spherical and cut into small hexagonal facets like the eyes of a fly. Grains of pollen of all kinds vary between one two-thousandth and one two-hundredth of an inch in diameter.
Alliances between plants and birds are more important than we imagine. The tropical Humming-birds and the eastern Sun-birds are in habits exactly like the pollen-carrying insects. To watch one of these brilliantly coloured creatures hovering over a flower or flying directly into a blossom after nectar, is to almost always mistake it for a Butterfly.
Many birds are invaluable allies of the plant world. They devour thousands of leaf-eating insects per day and so keep down the army of enemies which would otherwise destroy whole forests. Birds like the Woodpeckers rid tree bark of wood-boring crawlers.
In the human world every partner does not always live up to his agreements. And there are evidences that both plants and their allies sometimes engage in questionable practices, bordering on deception and chicanery.
The insects are often enough the offenders, and their crime is most frequently one of robbery. If they can get the sweets they are after without carrying out their share of the bargain, they will do so. Bumble Bees have been observed to cut through the flower-walls of a Nasturtium and so extract its nectar without coming near the pollen-producing stamens. Sweet Peas frequently ignore the insects and fertilize themselves. The Hawkweed (Hieracium) has so little faith in insect allies that it produces seeds parthenogenetically, that is, without the union of sex elements.
Alliances which start out advantageously for both parties sometimes degenerate into mere sinecures for one or the other. The naturalists Ihering, Ule and Fiebrig, working in South America, a few years ago concluded that the association of the plant Cecropia and the Aztecan Ants, long regarded as a classic example of mutualism, is by far of greater benefit to the Ants. The openings which the Ants make into the hollow interiors of this plant also allow the entrance of certain destructive insects, and the Ants themselves attract Woodpeckers which damage the plants. It is also alleged that these same Ants, and the ones which inhabit the Humboldtia Laurifolia, are often so busy feasting on nectar that they do not stop to repel invasions of foliage-destroying insects.
While man is the greatest enemy of the plant world, he is also at times its greatest friend. When it is to his advantage or when he is prompted by a sincere love of Nature, he becomes a strong and helpful ally. He aids his fellow creatures of the vegetable world when they are sick or injured and, by improving their environment and protecting them from attack and danger, enables them to develop to best advantage. A wizard like Luther Burbank helps them in their efforts at race improvement and development.
In Egypt and Arabia, man has acted as carrier of pollen for centuries, and has thus insured an abundant Date crop. The same thing is often done in other parts of the world with Apples, Pistachios, Melons, Cucumbers and other plants having unisexual flowers.