amount of water in the soil and air surrounding them. Such are most of our forest and fruit trees, and most of our garden vegetables. Conditions of moderate moisture are called mesophytic; the plants living thus are known as mesophytes.

It may easily be seen that plants which are mesophytes at one time may under some conditions of weather be forced to undergo xerophytic or hydrophytic conditions. An oak tree may receive. no water through the roots during the winter because the surface

of the ground is frozen, thus preventing water from finding its way below the surface; on the other hand, during excessive rains in the spring it might exist for a time under almost hydrophytic conditions. But many trees are annually killed in districts where lumbering is going on through the damming of streams and formation of artificial ponds, which increase the water supply of the trees near by and soon kill them.

Other Factors.

It is a matter of common knowledge that plants in different regions of the earth differ greatly from one another in shape, size, and general appearance. If we study the causes for

these changes, it becomes evident that the very same factor, water supply, which governs hydrophytic, xerophytic, and mesophytic conditions, determines, at least in part, the habits of the plants growing in a given region - be it in the tropics or arctic regions. But in addition to water supply, the factors of temperature, light, soil, wind, etc., all play important parts in determining the form and structure of a plant.

Cold Regions. Here plants, which in lowland regions of greater warmth and moisture have a tall form and luxuriant foliage, are stunted and dwarfed; the leaves are smaller and tend to gather in rosettes, or are otherwise closely placed for warmth and protection. As we climb a mountain we find that the average size of plants decreases as we approach the line of perpetual snow. The largest trees occur at the base of the mountains; the same species of trees near the summit appear as mere shrubs. Continued cold and high winds are evidently the factors which most influence the slow growth and the size and shape of plants near the mountain tops. Cold,

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 aërial roots.

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

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.