Parasites and Saprophytes.—A plant that is dependent on a living plant or animal is a parasite, and the plant or animal on which it lives is the host. The dodder is a true parasite; so are the rusts, mildews, and other fungi that attack leaves and shoots and injure them.
Fig. 133.—Dodder in Fruit.
The threads of a parasitic fungus usually creep through the intercellular spaces in the leaf or the stem and send suckers (or haustoria) into the cells (Fig. [132]). The threads (or the hyphæ) clog the air-spaces of the leaf and often plug the stomates, and they also appropriate and disorganize the cell fluids; thus they injure or kill their host. The mass of hyphæ of a fungus is called mycelium. Some of the hyphæ finally grow out of the leaf and produce spores or reproductive cells that answer the purpose of seeds in distributing the plant (b, Fig. [132]).
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]).
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.
Fig. 135.—Bacteria of Several Forms, much magnified.