meadow, along a roadside, or at the edge of a pond. Any one familiar with the country knows instinctively that we find certain plants, and those plants only, living together under certain conditions. For example, the wild columbine, certain ferns, and mosses, and other shade, moisture, and rock-loving plants are found together on rocky, shaded hillsides. We should not think of looking for daisies and buttercups there any more than we should look for

Plant societies near a pond. Notice that the plant groups are arranged in zones with reference to the water supply, the true mesophytes being in the background.

the marsh marigold (Caltha palustris) or the pickerel weed (Pontederia cordata) in a dry and sunny field.

Plants associated under similar conditions, as those of a forest, meadow, or swamp, are said to make up a formation, and a plant formation is brought about by the conditions of its immediate surroundings, the habitat of its members. If we investigate a plant formation, we find it to be made up of certain dominant species of plants; that here and there definite communities exist, made up of groups of the same kind of plants. We can see that every one of these plant groups in the society evidently originally came from single individuals of species which thrive under the peculiar conditions of soil, water, light, etc., that we find in this spot. These

single plants have evidently given rise to the members of each little family group, and thus have populated the locality.

So we find among plants communal conditions similar to those among some animals. The many individuals of the community live under similar conditions; they need the same substances from the air, the water, the soil. They all need the light; they use the same food. Therefore there must be competition among them, especially between those near to each other. The plants which are strongest and best fitted to get what they need from their surroundings live; the weaker ones are crowded out and die.

But their lives are not all competition. The dead plants and animals give nitrogenous material to the living ones, from which the

storm, while the trees hold the moisture in the ground, giving it off slowly to other plants. Animals scatter and plant the seeds far and wide, and man may even plant entire colonies in new localities.

How Plants invade New Areas. New areas are tenanted by plants in a similar manner. After the burning over of a forest, we find a new generation of plants springing up, often quite unlike the former occupants of the soil. First come the fireweed and other light-loving weeds, planted by means of their wind-blown seeds. With these are found patches of berries, the seeds of which were brought by birds or other animals. A little later, quick-growing trees having seeds easily carried for some distance by the wind,

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

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.

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.

The Economic Value of Plants. - Besides the other relations 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.

A

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.1 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.