REFERENCE BOOKS
ELEMENTARY
Hunter, _Laboratory Problems in Civic Biology_. American Book Company.
Gannet, _Commercial Geography_. American Book Company.
Sargent, _Plants and their Uses_. Henry Holt and Company.
Toothaker, _Commercial Raw Materials_. Ginn and Company.
U. S. Dept. of Agriculture, Farmers' Bulletin 86, _Thirty Poisonous Plants of the United States_, V. K. Chestnut.
Bulletin 17. _Two Hundred Weeds, How to Know Them and How to Kill Them_, L. H. Dewey.
ADVANCED
Bailey, _Cyclopedia of American Agriculture_. The Macmillan Company.
XI. PLANTS WITHOUT CHLOROPHYLL IN THEIR RELATION TO MAN
_Problems.--(a) How molds and other saprophytic fungi do harm to man._ _(b) What yeasts do for mankind._ _(c) A study of bacteria with reference to_ _(1) Conditions favorable and unfavorable to growth._ _(2) Their relations to mankind._ _(3) Some methods of fighting harmful bacteria and diseases caused by them._
LABORATORY SUGGESTIONS
_Field work._--Presence of bracket fungi and chestnut canker.
_Home experiment._--Conditions favorable to growth of mold.
_Laboratory demonstration._--Growth of mold, structure, drawing.
_Home experiment or laboratory demonstration._--Conditions unfavorable for growth of molds.
_Demonstration._--Process of fermentation.
_Microscopic demonstration._--Growing yeast cells. Drawing.
_Home experiment._--Conditions favorable for growth of yeast.
_Home experiment._--Conditions favorable for growth of yeast in bread.
_Demonstration and experiment._--Where bacteria may be found.
_Demonstration._--Methods of growth of bacteria, pure cultures and colonies shown.
_Demonstration._--Foods preferred by bacteria.
_Demonstration._--Conditions favorable for growth of bacteria.
_Demonstration._--Conditions unfavorable for growth of bacteria.
_Demonstration by charts, diagrams, etc._--The relation of bacteria to disease in a large city.
COLORLESS PLANTS ARE USEFUL AND HARMFUL TO MAN
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 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.
YEASTS IN THEIR RELATION TO MAN
Fermentation.--It is of common knowledge to country boys or girls that the juice of fresh apples, grapes, and some other fruits, if allowed to stand exposed to the air for a short time will _ferment_. That is, the sweet juice will begin to taste sour and to have a peculiar odor, which we recognize as that of alcohol. The fermenting juice appears to be full of bubbles which rise to the surface. If we collect enough of these bubbles of gas to make a test, we find it to be carbon dioxide.
Evidently something changed some part of the apple or grape, the sugar, (C{6}H{12}O{6}), into alcohol, 2(C{2}H{6}O), and carbon dioxide, 2(CO{2}).
This chemical process is known as _fermentation_.
[Illustration: Apparatus to show effect of fermentation. _N_, molasses, water and yeast plants; _C_, bubbles of carbon dioxide.]
Yeast causes Fermentation.--Let us now take a compressed yeast cake, shake up a small portion of it in a solution of molasses and water, and fill a fermentation tube with the mixture. Leave the tube in a warm place overnight. In the morning a gas will be found to have been collected in the closed end of the tube (see Figure on page 138). The taste and odor of the liquid shows alcohol to be present, and the gas, if tested, is proven carbon dioxide. Evidently yeast causes fermentation.
What are Yeasts?--If now part of the liquid from the fermentation tube which contains the settlings be drawn off, a drop placed on a slide and a little weak iodine added and the mixture examined under the compound microscope, two kinds of structures will be found (see Figure below), starch grains which are stained deep blue, and other smaller ovoid structures of a brownish yellow color. The latter are yeast plants.
[Illustration: Yeast and starch grains. Notice that the starch grains around which are clustered yeast cells have been rounded off by the yeast plants. How do you account for this?]
Size and Shape, Manner of Growth, etc.--The common compressed yeast cake contains millions of these tiny plants. In its simplest form a yeast plant is a single cell. The shape of such a plant is ovoid, each cell showing under the microscope the granular appearance of the protoplasm of which it is formed. Look for tiny clear areas in the cells; these are vacuoles, or spaces filled with fluid. The nucleus is hard to find in a yeast cell. Many of the cells seem to have others attached to them, sometimes there being several in a row. Yeast cells reproduce very rapidly by a process of budding, a part of the parent cell forming one or more smaller daughter cells which eventually become free from the parent.