Some parasites spring from the ground, as other plants do, but they are _parasitic on the roots of their hosts_. Some parasites may be _partially parasitic_ and _partially saprophytic_. Many (perhaps most) of these ground saprophytes are aided in securing their food by soil fungi, which spread their delicate threads over the root-like branches of the plant and act as intermediaries between the food and the saprophyte. These fungus-covered roots are known as =mycorrhizas= (meaning “fungus root”). Mycorrhizas are not peculiar to saprophytes. They are found on many wholly independent plants, as, for example, the heaths, oaks, apples, and pines. It is probable that the fungous threads perform some of the offices of root-hairs to the host. On the other hand, the fungus obtains some nourishment from the host. The association seems to be mutual. [Illustration: FIG. 135.--BACTERIA OF SEVERAL FORMS, much magnified.] Saprophytes break down or decompose organic substances. Chief of these saprophytes are many microscopic organisms known as bacteria (Fig. 135). These innumerable organisms are immersed in water or in dead animals and plants, and in all manner of moist organic products. By breaking down organic combinations, _they produce decay_. Largely through their agency, and that of many true but microscopic fungi, _all things pass into soil and gas_. Thus are the bodies of plants and animals removed and the continuing round of life is maintained. [Illustration: FIG. 136.--AMERICAN MISTLETOE GROWING ON A WALNUT BRANCH.] _Some parasites are green-leaved._ Such is the mistletoe (Fig. 136). They anchor themselves on the host and absorb its juices, but they also appropriate and use the carbon dioxid of the air. In some small groups of bacteria a process of organic synthesis has been shown to take place. =Epiphytes.=--To be distinguished from the dependent plants are those that grow on other plants without taking food from them. These are green-leaved plants whose roots burrow in the bark of the host plant and perhaps derive some food from it, but which subsist chiefly on materials that they secure from air dust, rain water, and the air. These plants are =epiphytes= (meaning “upon plants”) or air plants. Epiphytes abound in the tropics. Certain orchids are among the best known examples (Fig. 37). The Spanish moss or tillandsia of the South is another. Mosses and lichens that grow on trees and fences may also be called epiphytes. In the struggle for existence, the plants probably _have been driven to these special places_ in which to find opportunity to grow. Plants grow where they must, not where they will.