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like the aspen and wild cherry, which have the birds to help them out, invade the territory. Eventually we may have the area retenanted by its former inhabitants, especially if the destruction of the original forest was not complete.

In like manner, on the upper mountain meadow or by the sand dunes of the seashore, wherever plants place their outposts, the

[graphic]

A plant outpost. The struggle here is keen. The advancing sand has
killed the trees in the foreground.

advance is made from some thickly inhabited area, and this advance is always aided or hindered by agencies outside of the plant - the wind, the soil, water, or by animals. Thus the seeds obtain a foothold in new territory, and thus new lands are captured, held, and lost again by the plant communities.

REFERENCE BOOKS

ELEMENTARY

Sharpe, A Laboratory Manual for the Solution of Problems in Biology. American Book Company.

Andrews, Botany All the Year Round. American Book Company.

Bergen and Davis, Principles of Botany. Ginn and Company.

Coulter, Plant Relations. D. Appleton and Company.

Leavitt, Outlines of Botany. American Book Company.

Stevens, Introduction to Botany. D. C. Heath and Company.

ADVANCED

Clements, Plant Physiology and Ecology. Henry Holt and Company.

Coulter, Barnes, and Cowles, A Textbook of Botany, Vol. II. American Book Com

pany.

Kerner, Natural History of Plants. 4 vols. Henry Holt and Company.

Schimper, Plant Geography. Clarendon Press.

XIII. HOW PLANTS BENEFIT AND HARM MANKIND

Problem XXII. The relations of fungi to man. (Laboratory Manual, Prob. XXII.)

(a) Yeast.

(b) Other fungi.

Besides the other relations

The Economic Value of Plants. existing between plants and animals, there is a relation between man and plants measurable in dollars and cents. Plants are of direct value or harm to man. We call this an economic relation. We have seen how they supply him with his cereals and flour, his fruits and garden vegetables, his nuts and spices, his beverages and the sugar to sweeten them, his medicines and his dyestuffs. They supply the material out of which many of his clothes are made, the thread with which they are sewed together, the paper which covers the package in which they are delivered, and the string with which the package is tied. The various uses of the forest have been mentioned before; the need of trees to protect the earth, their usefulness in the holding of the water supply, their direct economic importance for lumber and firewood. Many of us forget, too, that much of the energy released on this earth to man as heat, light, or motive power comes from the dead and compressed bodies of plants which thousands of years ago lived on the earth and now form coal. Plants are thus seen to be of immense direct economic importance to mankind.

The Harm Plants Do. — Unfortunately, plants do not all benefit mankind. We have seen the harm done by weeds, which scatter their numerous seeds far and wide or by other devices gain a foothold and preempt the territory which useful plants might occupy were they able to cope with their better-equipped adversaries. Plants with poisonous seeds and fruits are undoubtedly responsible for the death of many animals.

But by far the most harmful plants to mankind are the fungi.

Hundreds of millions' yearly damage may be laid directly to them. More than that, they are doubtless responsible for one half of the total human deaths. This is because of their parasitic habits.

Yeast. Although as a group the fungi are harmful to man in the economic sense, nevertheless there are some fungi that stand in a decidedly helpful relationship to the human race. Chief

of these are the yeast plants. Yeasts are found to exist in a wild state in very many parts of the world. They are found on the skins of fruits, in the soil of vineyards and orchards, in cider, beer, and other fluids, while they may exist in a dry state almost anywhere in the air around us. In a cultivated state we find them doing our work as the agents which cause the rising of bread, and the fermentation in beer and other alcoholic fluids.

B

A, yeast plant bud just forming; B, bud almost ready to leave parent cell. Note the nucleus (N) dividing into two parts. (After Sedgwick and Wilson.)

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. If you shake up a bit of a compressed yeast cake in a mixture of sugar and water and then examine a drop of the milky fluid after it has stood overnight, it will be seen to contain vast numbers of yeast plants. 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 an unstained yeast cell; it can, however, be found in specimens which have been prepared by staining the previously killed cells with iron-hæmatoxylin. 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.

Most yeast plants seem to produce spores at some time during their existence. The spores are formed within a yeast cell, as many as four being produced within a single cell. These spores, under proper conditions, will germinate and give rise to new plants.

Conditions favorable to Growth of Yeast. Under certain conditions yeast, when added to dough, will cause it to rise. We also know that yeast has something to do with the process we call fermentation. The following home experiment will throw some light on these points:

Label three pint fruit jars A, B, and C. Add one fourth of a compressed yeast cake to two cups of water containing two tablespoonfuls of molasses or sugar. Stir the mixture well and divide it into three equal

1 See Lee, Vade Mecum, or Sedgwick and Wilson, General Biology.

parts and pour them into the jars. Place covers on the jars. Put jar A in the ice box on the ice, and jar B over the kitchen stove or near a radiator; boil the jar C by immersing it in a dish of boiling water, and place it next to B. After forty-eight hours, look to see if any bubbles have made their appearance in any of the jars. If the experiment has been successful only jar B will show bubbles. After bubbles have begun to appear at the surface, the fluid in jar B will be found to have a sour taste and will smell unpleasantly. The gas which rises to the surface, if collected and tested, will be found to be carbon dioxide. The contents of jar B are said to have fermented. Evidently, the growth of yeast will take place only under conditions of moderate warmth and moisture.

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Fermentation a Chemical Process. In this process of growth the sugar of the solution in which they live is broken up by a digestive ferment or enzyme into carbon dioxide and alcohol. This may be expressed by the following chemical formula: C6H12O6 2(C2H6O) + 2(CO2). This means that the sugar forms alcohol and carbon dioxide. This process, which we call fermentation, is of the greatest importance in the brewing industry.

Beer-Making. - Brewers' yeasts are cultivated with the greatest care; for the different flavors of beer seem to depend largely upon the condition of the yeast plants. Beer is made in the following manner: Sprouted barley, called malt, in which the starch of the grain has been changed to grape sugar by digestion, is killed by drying in a hot kiln. The malt is dissolved in water, and hops are added to give the mixture a bitter taste. Now comes the addition of the yeast plants, which multiply rapidly under the favorable conditions of food and heat. Fermentation results on a

large scale from the breaking down of the grape sugar, the alcohol remaining in the fluid, and the carbon dioxide passing off into the air. The process is stopped at the right instant, and the beer is stored either in bottles or casks.

Bread-Making. - In bread-making the rapid growth of the yeast plants is facilitated by placing the pan containing the mixture in a warm place overnight. Fermentation results from the digestion of grape sugar by the yeasts, this grape sugar being part of the starch in the flour which is changed by the diastase present in the grain of wheat. The carbon dioxide remains in the dough as the bubbles so familiar to the breadmaker, the alcohol produced being evaporated during the process of baking.

Yeast Saprophytes. The above paragraphs show yeast plants to be saprophytes. In order to grow, they must be supplied with food materials that will build up protoplasm as well as release energy. This food they obtain from the organic matter in the fluids in which they happen to be.

The Shelf Fungus; a Saprophyte. A near relation to the mushroom is the bracket or shelf fungus. This fungus is familiar to any one who has been in a forest in this part of the country.

An examination of specimens shows that the shelf or bracket is in reality a spore case, which is usually provided with a very considerable number of holes, slits, or pores in which the spores are formed. The spores when ripe escape from the under surface of the spore-bearing body through the minute pores. The mycelium is within the tissue of the tree. Remove the bark from any tree infected with bracket fungus, and you will find the silvery threads of the mycelium sending their greedy hyphæ to all parts of the wood adjacent to the spot first attacked by the fungus. This fungus begins its life by the lodgment of a spore in some part of the tree which has become diseased or broken. Once established on its host, 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 or crib on the tree, thus exposing a fresh surface on which spores may obtain lodgment and grow (see page 142).

[graphic]

Shelf or bracket fungi on dead tree trunk.

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?

Parasitic Fungi.

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Of even more importance are the fungi that attack a living host, true parasites. The most important of such

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