Literally "air plants:" those plants that do not grow in earth or water, but are supported in air on trees or other objects and usually drawing no organic nourishment from such object or support.

True epiphytes are widely distributed in all climates, but it is in the moist tropics that they become so numerous and conspicuous as to arouse the special interest and enthusiasm of the serious student as well as of the traveler or casual observer. One thinks of epiphytes as growing upon trees, and trees are usually the supporting plants. The term merely signifies that ecological type that has the habit of growing upon other plants, although in this account it is not the purpose to discuss such seaweeds or other algae as grow upon larger plants in the water. The word epiphyte also involves a contrast with parasite, the latter denoting that nourishment and water are derived from the living tissues of the supporting plant or host. The epiphytic habit implies no particular method of nutrition, and the epiphytes are entirely independent of the nutrition of the supporting plant. This habit is not restricted to a single class, or to a few families of plants, although in some families many representatives of the type have been developed, while in related families there may be none.

The seed plants are represented by many species of tropical orchids, arums, bromeliads, and numerous others; lycopods, ferns, mosses and liverworts all contribute many examples; and in the lower groups of plants the lichens are in some regions dominantly epiphytic.

The luxuriant tropical rain-forest is regarded as the climax in development of vegetation. In describing this type, Humboldt declared that "forest is piled upon forest." Under such conditions the trunks and branches are clothed with larger epiphytes, and the leaves of some species accommodate algae and lichens. It is in the South American tropical forests that the better known of our greenhouse epiphytes are native. Orchids, bromeliads, and arums are among the most abundant. In the Javanese forests, the wealth of species is great, but mosses, ferns and lycopods are particularly numerous, and these are accompanied by some interesting species of Ficus, epiphytic for a time, and by the striking Rhododendron javanicum, among others. In the mountain forests of tropical regions there are, as epiphytes, representatives of several families of ferns, likewise many mosses and lichens. The dicotylous and certain coniferous forests of Europe and America harbor a few mosses and liverworts and numerous species of lichens. A conspicuous epiphyte of the southern states, as well as of tropical America is the long or Florida moss, Tillandsia usneoides, the extremest epiphyte among the Bromeliacese. Accompanying this, the common polypody fern is also found on trees.

Going northward, the total number of epiphytic lichens may decrease, but several of the larger forms seem to become more abundant and some of the moss-like usneas extend to the northernmost latitude of tree growth.

The habit of growing upon trees renders epiphytes subject to an inconstant water-supply. On this account the larger and more delicate epiphytes are restricted to regions constantly moist. Even in the moist forest, the species less resistant to drying out are found on the lower branches, and those more resistant maintain themselves higher up, so that there is a distribution in strata, analogous to the lateral distribution of species about the edge of a pond. In general, however, there is exposure to drying out, and, as might be anticipated, these plants exhibit the structural characteristics of xerophytes (dryland plants). Many of them are modified so that transpiration is reduced, and they are able to withstand considerable desiccation. Among greenhouse forms this is notably true of many orchids and lichens. Moreover, many species of orchids possess special tissues to which water is transported and there accumulated as a "reserve" supply. Leaf-tissues may function in this way, but usually more important are the bulb-like enlargements of the stems.

Of special interest are the organs of absorption of certain epiphytes. Aerial roots are characteristic of tropical arums and orchids. The typical air-root is provided with an outer cylinder of tissue, the velamen, derived from the epidermis, consisting at maturity of dead cells capable of taking up liquid water and substances in solution like a sponge. From these roots as capillary reservoirs, the supply is gradually absorbed by the living tissues. Rain, dew, or moist substrata may furnish the water, but the view that these roots absorb water vapor is erroneous. The Bromeliaceae are peculiar in the possession of certain absorbing leaf-scales or hairs. The Florida moss possesses such hairs over the entire surfaces of the thread-like stems and leaves, and the plant is rootless. There are all gradations between this and the soil-rooted pineapple - like forms. The arrangement of the leaves in many of the bromeliads possessing larger leaves is such as to establish after a rain a temporary reservoir about the leaf - bases.

The absorbing scales of the bromeliads exhibit features worthy of note in three particulars: (1) When dry certain dead cells absorb water greedily; (2) with absorption they assume a position making possible the entry of water to a considerable surface of living cells, and (3) with collapse, due to loss of water, the spaces admitting water are closed and loss is minimized.

Aside from such saprophytic fungi as might be considered epiphytic, the epiphytes are amply provided with chlorophyll - bearing tissue; therefore, organic food is manufactured as in other plants. Some of the epiphytes growing upon such humus-developing sub-trata as the decaying bark of trees, or such as passively accumulate humus and other materials in the vicinity of their absorbing surfaces, might absorb some organic compounds as well as salts in this way; but this supply of organic matter is certainly inconsequential in most cases. Water and salts are secured either through the air-roots, as described, or partially through normal roots, when such occur. Many species, epiphytic at first, ultimately send roots into the soil, and then secure water and salts largely through the terrestrial habit.

In the forest, certain of the seed-bearing epiphytes are specialized with respect to supporting plants, often due to the special nature of the protection offered, or to the physical advantages of the substratum in regard to fixation of the plant. Most species are markedly unspecialized and may be grown in the greenhouse most successfully. B. M. Duggar.