DEPENDENT PLANTS
Thus far we have spoken of plants that have 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 dioxide. 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.
[Illustration: 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).
[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.
=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.