Plants living in water have loose and spongy tissues; many large intercellular spaces are found in stems or leaves. In one pond lily (Nelumbo lutea) these spaces in the leaf communicate with large spaces in the veins of the leaf, and these in turn with spaces in the petiole, stem, and root, so that all parts of the plants are in communication with the air above. The roots of a plant living wholly in water are not needed for support, hence they are often short and stumpy. They do not need to be modified to absorb water; consequently the absorbing surface lacks root hairs. The whole plant, when under water, is usually modified to take water (and with it food) from its immediate environment.

Hydrophytes. If water is present in such quantity as to saturate the soil in which the plant lives the conditions of its en

vironment are said to be hydrophytic and such plant is said to be a hydrophyte.

Xerophytes. The opposite

of hydrophytic conditions is seen when the soil is very dry. Such a condition is known as xerophytic, and the plants living in these conditions are xerophytes. Such is the condition in a desert. We have seen that the most important adaptations of xerophytes are such as prevent evaporation of water from their bodies. The leaf surface is reduced, the leaves being changed into spines as in the cactus, or very greatly reduced in size, as in the switch plants of our alkali

deserts. The stem may be thickened and full of water; a covering of hairs or some other covering may occur and lessen loss of moisture by evaporation. Examples of xerophytes are the cacti, yuccas, agaves, etc.

Halophytes. If the water or saturated soil in which the plant lives contains salts, such as sea salt or the alkali salts of some of our Western lakes, then the conditions are said to be halophytic, and a plant living under such conditions is known as a halophyte.

Halophytes show many characteristics which xerophytes show, spines or hairs, thick epidermis, fleshy leaves, all being characters which show that the water supply of the plant is limited. The density of the salt water in the soil makes it difficult for the plant to absorb water; hence these characters are developed.

Mesophytes. Most plants in the Temperate Zone occupy a place midway between the xerophytes on one hand and hydrophytes on the other. They are plants which require a moderate amount of water in the soil and air surrounding them. Such are most of our forest and fruit trees, and many 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 HUNTER'S BIOL.-10

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.1

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 factors which govern hydrophytic, xerophytic, and mesophytic conditions determine, 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.

1 Suggestions for such excursions are found in Andrews, Botany all the Year Round, Lloyd and Bigelow, The Teaching of Biology, Ganong, The Teaching Botanist, and many other books. A convenient form of excursion is found in Hunter and Valentine, Manual, page 202.

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. Trees over five hundred years old have been noted in cold regions with trunks less than three feet in diameter at the base.

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

VEGETATION OF THE TROPICS. - A rank and luxuriant growth is found in tropical countries with a uniformly high temperature and large rainfall. In general it may be estimated that the rainfall in such countries is at least twice as great as that of New York state, and in many cases three to four times as great. An abundant water 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 of the growth of many air plants (epiphytes), which take the moisture directly from the air by means of aërial roots.