The Fungi.--We have found that green plants on the whole are useful to
mankind. But not all plants are green. Most of us are familiar with the
edible mushroom sold in the markets or the so-called "toadstools" found in
parks or lawns. These plants contain no chlorophyll and hence do not make
their own food. They are members of the plant group called _fungi_. Such
plants are almost as much dependent upon the green plants for food as are
animals. But the fungi require for the most part dead organic matter for
their food. This may be obtained from decayed vegetable or animal material
in soil, from the bodies of dead plants and animals, or even from foods
prepared for man. Fungi which feed upon _dead_ organic material are known
as _saprophytes_. Examples are the mushrooms, the yeasts, molds, and some
bacteria, of which more will be learned later.
[Illustration: Chestnut trees in a New York City park; killed by a
parasite, the chestnut canker.]
Some Parasitic Fungi.--Other fungi (and we will find this applies to some
animals as well) prefer _living_ plants or animals for their food. Thus a
tiny plant, recently introduced into this country, known as the chestnut
canker, is killing our chestnut trees by the thousands in the eastern part
of the United States. It produces millions of tiny reproductive cells known
as _spores_; these spores, blown about by the wind, light on the trees,
sprout, and send in under the bark a threadlike structure which sucks in
the food circulating in the living cells, eventually causing the death of
the tree. _A plant or animal which lives at the expense of another living
plant or animal is called a parasite._ The chestnut canker is a dangerous
parasite. Later we shall see that animal and plant parasites destroy yearly
crops and trees valued at hundreds of millions of dollars and cause untold
misery and suffering to humanity.
[Illustration: Shelf fungi. (Photographed by W. C. Barbour.)]
Another fungus which does much harm to the few trees found in large towns
and cities is the shelf or bracket fungus. The part of the body visible on
the tree looks like a shelf or bracket, hence the name. This bracket is in
reality the reproductive part of the plant; on its lower surface are formed
millions of little bodies called _spores_. These spores are capable, under
favorable conditions, of reproducing new plants. The true body of the
plant, a network of threads, is found under the bark. This fungus begins
its life as a spore in some part of the tree which has become _diseased_ or
_broken_. Once established, it spreads rapidly. There is no remedy except
to kill the tree and burn it, so as to destroy the spores. Many fine trees,
sound except for a slight bruise or other injury, are annually infected and
eventually killed. In cities thousands of trees become infected through
careless hitching of horses so that the horse may gnaw the tree, thus
exposing a fresh surface on which spores may obtain lodgment and grow (see
page 115).
Suggestions for Field Work.--A field trip to a park or grove near home may
show the great destruction of timber by this means. Count the number of
perfect trees in a given area. Compare it with the number of trees attacked
by the fungus. Does the fungus appear to be transmitted from one tree to
another near at hand? In how many instances can you discover the point
where the fungus first attacked the tree?
Fungi of our Homes.--But not all fungi are wild. Some have become
introduced into our homes and these live on food or other materials. _These
plants are very important because of their relation to life in a town or
crowded city._[17]
Footnote 17: Experiments on conditions favorable to growth
of mold should be introduced here.
[Illustration: Bread mold; _r_, rhizoids; _s_, fruiting bodies containing
spores.]
The Growth of Bread Mold.--If a piece of moist bread is exposed to the air
of the schoolroom, or in your own kitchen for a few minutes and then
covered with a glass tumbler and kept in a warm place, in a day or two a
fuzzy whitish growth will appear on the surface of the bread. This growth
shortly turns black. If we now examine a little piece of the bread with a
lens or low-powered microscope, we find a tangled mass of threads (the
_mycelium_) covering the surface of the bread. From this mass of threads
project tiny upright stalks bearing round black bodies, the fruit. Little
rootlike structures known as _rhizoids_ dip down into the bread, and absorb
food for its threadlike body. The upright threads with the balls at the end
contain many tiny bodies called _spores_. These spores have been formed by
the division of the protoplasm making up the fruiting bodies into many
separate cells. When grown under favorable conditions, the spores will
produce more mycelia, which in turn bear fruiting bodies.
Physiology of the Growth of Mold.--Molds, in order to grow rapidly, need
oxygen, moisture, and moderate heat. They seem to prefer dark, damp places
where there is not a free circulation of air, for if the bell jar is
removed from growing mold for even a short time, the mold wilts. Too great
or very little heat will prevent growth and kill everything except the
spores. They obtain their food from the material on which they live. This
they are able to do by means of digestive enzymes given out by the rootlike
parts, by means of which the molds cling to the bread. These digestive
enzymes change the starch of the bread to sugar and the protein to a
soluble form which will pass by osmosis into cells of the mold. Thus the
mold is able to absorb food material. These foods are then used to supply
energy and make protoplasm. This seems to be the usual method by which
saprophytes make use of the materials on which they live.
What can Molds live On?--We have seen that black mold lives upon bread. We
would find that it or some other mold (_e.g._ green or blue mold) live upon
decaying or overripe fruit,--apples, peaches, and plums being especially
susceptible to their growth. Molds feed upon all cakes or breads, upon
meat, cheese, and many raw vegetables. They are almost sure to grow upon
flour if it is allowed to get damp. Moisture seems necessary for their
growth. Jelly is a substance particularly favorable to molds for this
reason. Shoes, leather, cloth, paper, or even moist wood will give food
enough to support their growth. At least one troublesome disease,
_ringworm_, is due to the growth of molds in the skin.
What Mold does to Foods.--Mold usually changes the taste of the material it
grows upon, rendering it "musty" and sometimes unfit to eat. Eventually it
will spoil food completely because decay sets in. Decay, as we will see
later, is not entirely due to mold growth, but is usually caused by another
group of organisms, the _bacteria_. Molds, however, in feeding _do_ cause
chemical changes which result in decay or putrefaction. Some molds are
useful. They give the flavor to Roquefort, Gorgonzola, Camembert, and Brie
cheeses. But on the whole molds are pests which the housekeeper wishes to
get rid of.
How to prevent Molds.[18]--As we have seen, moisture is favorable for mold
growth; conversely, dryness is unfavorable. Inasmuch as the spores of mold
abound in the air, materials which cannot be kept dry should be covered.
Jelly after it is made should at once be tightly covered with a thin layer
of paraffin, which excludes the air and possible mold spores. Or waxed
paper may be fastened over the surface of the jelly so as to exclude the
spores. To prevent molds from attacking fresh fruit, the surface of the
fruit should be kept dry and, if possible, each piece of fruit should be
wrapped in paper. Why? Heating with dry heat to 212 deg. for a few moments will
kill any mold spores that happen to be in food. Moldy food, if heated after
removing surface on which the mold grew, is perfectly good to eat.
Footnote 18: An experiment to show conditions unfavorable
for growth of molds should be shown at this point.
Dry dusting or sweeping will raise dust, which usually contains mold
spores. Use a dampened broom or dust cloth frequently in the kitchen if you
wish to preserve foods from molds.
Other Moldlike Fungi.--Mildews are near relatives of the molds found in our
homes. They may attack leather, cloth, etc., in a damp house. Other allied
forms may do damage to living plants. Some of these live upon the lilac,
rose, or willow. These fungi do not penetrate the host plant to any depth,
for they obtain their food from the outer layer of cells in the leaf of
their host and cover the leaves with the whitish threads of the mycelium.
Hence they may be killed by means of applications of some fungus-killing
fluid, as Bordeaux mixture.[19] Among the useful plants preyed upon by
mildews are the plum, cherry, and peach trees. (The diseases known as black
knot and peach curl are thus caused.) Another important member of this
group is the tiny parasite found on rye and other grains, which gives us
the drug ergot.
Footnote 19: See Goff and Mayne, _First Principles of
Agriculture_, page 59, for formula of Bordeaux mixture.
Among other parasitic fungi are rusts and smuts. Wheat rust is probably the
most destructive parasitic fungus. Indirectly this parasite is of
considerable importance to the citizen of a great city because of its
effect upon the price of wheat.