little light during the short days of the long winter, and a slight amount of moisture all act upon the vegetation of the arctic region, tending toward very slow growth and dwarfed and stunted form.

Polar limit of trees, northern Russia. All these trees are full grown, and most of them are almost one hundred years old.

Vegetation of the Tropics. A rank and luxuriant growth is found in tropical countries with a uniformly high temperature and

Plant regions in a tropical mountain. Explain the diagram. supply, together

with an average temperature of over 80° Fahrenheit, causes extremely rapid growth. One of the bamboo family, the growth of which was measured daily, was found to increase in length on the average nearly three inches in the daytime and over five inches during each night. The moisture present in the atmosphere allows the growth of many air plants (epiphytes), which take the moisture directly from the air by means of aëral roots.

Conditions in a moist, semitropical forest. The so-called "Florida moss" is a flowering plant. Notice the resurrection ferns on the tree trunk.

The absence of cold weather in tropical countries allows trees to mature without a thick coating of bark or corky material, plants all having a green and fresh appearance. Monocotyledonous plants prevail. Ferns of all varieties, especially the largest tree ferns, are abundant.

Plant Life in the Temperate Zones. In the state of New York, conditions are those of a typical temperate flora. Extremes of cold and heat are found, the temperature ranging from 30° Fahrenheit below zero in the winter to 100° or over in the summer. Conditions of moisture show an average rainfall of from 24 inches to 52

inches. Conditions of moisture in the country cause great differences in the plant covering.

In the eastern part of the United States the rainfall is sufficient to give foothold to great forests, which aid in keeping the water in the soil. In the Middle West the rainfall is less, the prairies are covered with grasses and other plants which have become adapted to withstand dryness. In the desert region of the Southwest we find true xerophytes, cacti, switch plants, yuccas, and others, all

A rock society. Photograph by W. C. Barbour.

plants which are adapted to withstand almost total absence of moisture for long periods. In the Temperate Zone the water supply is the primary factor which determines the form of plant growth. Plant Formations and Societies.1 All of the factors alluded to act upon the plants we find living together in a forest, a sunny

1 Plant Societies. Field Work. - Any boy or girl who has access to a vacant lot or city park can easily see that plants group themselves into societies. Certain plants live together because they are adapted to meet certain conditions. Societies of plants exist along the dusty edge of the roadside, under the trees of the forest, along the edge of the brook, in a swamp or a pond. It should be the aim of the field trips to learn the names of plants which thus associate themselves and the conditions under which they live, and especially their adaptations to the given conditions. Suggestions for such excursions are found in Andrews, Botany All the Year Round; Lloyd and Bigelow, The Teaching of Biology; and Ganong, The Teaching Botanist. A convenient form for an excursion is found in Hunter and Valentine, Manual, page 202. This trip may be taken in the early fall.

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,