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.