A plant that lives on dead or decaying matter is a =saprophyte=. Mushrooms (Fig. 131) are examples; they live on the decaying matter in the soil. Mould on bread and cheese is an example. Lay a piece of moist bread on a plate and invert a tumbler over it. In a few days it will be mouldy. The spores were in the air, or perhaps they had already fallen on the bread but had not had opportunity to grow. Most green plants are unable to make any direct use of the humus or vegetable mould in the soil, for they are not saprophytic. The shelf fungi (Fig. 134) are saprophytes. They are common on logs and trees. Some of them are perhaps partially parasitic, extending the mycelium into the wood of the living tree and causing it to become black-hearted (Fig. 134). [Illustration: FIG. 134.—TINDER FUNGUS (_Polyporus igniarius_) on beech log. The external part of the fungus is shown below; the heart-rot injury above.] 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 dioxide of the air. In some small groups of bacteria a process of organic synthesis has been shown to take place.