With this change in habit came a need of taking in water, of storing it, of conducting it to various parts of the organism. So it does not seem unlikely that plants came to have roots, stems, and leaves and thus adapted to their environment on dry land. We find in nature that those plants or animals which are best adapted or fitted to live under certain conditions are the ones which survive or drive other competitors out from their immediate neighborhood. Nature selected those which were best fitted to live on dry land, and those plants eventually covered the earth with their progeny.

As we have found in our experiments, young plants, and indeed any living plants, are delicate organisms, which are affected profoundly by the action of forces outside themselves. It is impossible not to see this after we have grown seedlings with and without light, with much water and with little water. Pea seedlings may grow for a time in sawdust, but we know that they will be much healthier and will live longer if allowed to germinate in soil under natural conditions.

Desert Conditions. If we examine plants growing in a dry climate, as cactus, sage brush, aloe, etc., we find that the leaf surface is invariably reduced. Leaves are reduced to spines in the cactus. Some plants, such as the three-angled spurge, which bear leaves in a condition of moderate water supply, take on the appearance of a cactus under desert conditions. Thus they lose their evaporating leaf surface by having the leaves changed into spines.

This adaptation is evidently, if our experiments count for anything, the result of the action of forces outside the plants; that is, it is an adaptation to environment.

Water Supply. - Water supply is one of the important factors in causing changes in structure of plants. Plants which live entirely in the water, as do many of the plants known as algæ, have slender parts, stemlike, and yet serving the place of a leaf. The interior of such a plant is made up of spongy tissues which allow the air, dissolved in the water in which they live, to reach them. If leaves are present, as in the pond lily, the stomata are all in the upper side of the leaf.

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.