CHAPTER XIV

DEPENDENT PLANTS

Thus far we have spoken of plants with roots and foliage and that depend on themselves. They collect the raw materials and make them over into assimilable food. They are =independent=. Plants without green foliage cannot make food; they must have it made for them or they die. They are =dependent=. A sprout from a potato tuber in a dark cellar cannot collect and elaborate carbon dioxid. It lives on the food stored in the tuber.

[Illustration: FIG. 131.--A MUSHROOM, example of a saprophytic plant. This is the edible cultivated mushroom.]

_All plants with naturally white or blanched parts are dependent._ Their leaves do not develop. They live on organic matter--that which has been made by a plant or elaborated by an animal. The dodder, Indian pipe, beech drop, coral root among flower-bearing plants, also mushrooms and other fungi (Figs. 131, 132) are examples. The dodder is common in swales, being conspicuous late in the season from its thread-like yellow or orange stems spreading over the herbage of other plants. One kind attacks alfalfa and is a bad pest. The seeds germinate in the spring, but as soon as the twining stem attaches itself to another plant, the dodder dies away at the base and becomes wholly dependent. It produces flowers in clusters and seeds itself freely (Fig. 133).

[Illustration: FIG. 132.--A PARASITIC FUNGUS, magnified. The mycelium, or vegetative part, is shown by the dotted-shaded parts ramifying in the leaf tissue. The rounded haustoria projecting into the cells are also shown. The long fruiting parts of the fungus hang from the under surface of the leaf.]

=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.

The threads of a parasitic fungus usually creep through the intercellular spaces in the leaf or 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).

[Illustration: FIG. 133.--DODDER IN FRUIT.]

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. Mold 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 moldy. 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 mold 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 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.

SUGGESTIONS.--=114.= Is a puffball a plant? Why do you think so? =115.= Are mushrooms ever cultivated, and where and how? =116.= In what locations are mushrooms and toadstools usually found? (There is really no distinction between mushrooms and toadstools. They are all mushrooms.) =117.= What kinds of mildew, blight, and rust do you know? =118.= How do farmers overcome potato blight? Apple scab? Or any other fungous “plant disease”? =119.= How do these things injure plants? =120.= What is a plant disease? =121.= The pupil should know that every spot or injury on a leaf or stem is caused by something,--as an insect, a fungus, wind, hail, drought, or other agency. How many uninjured or perfect leaves are there on the plant growing nearest the schoolhouse steps? =122.= Give formula for Bordeaux mixture and tell how and for what it is used.