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.