XII. HOW PLANTS ARE MODIFIED BY THEIR SUR

ROUNDINGS

Problem XXI. How plants are modified by their surroundings. (Optional). (Laboratory Manual, Prob. XXI.)

(a) Hydrophytic society.

(b) Xerophytic society.

(c) Mesophytic society. (d) Plant societies.

(e) Plant zonation.

The Way in which Plants are Modified by their Surroundings. 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. The presence or absence of moisture starts or prevents growth in seeds or young plants; absence of light changes the form and color of green plants; a certain temperature, which varies for different plants, seems to influence plants in a healthy growth. 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. We are forced to the conclusion that differences in the form and habits of plants are caused by the action of their surroundings upon them. The plants which have become in various ways fitted

Pond lilies, plants with floating leaves. Photograph by W. C. Barbour.

to live under certain conditions are said to be adapted to live under such conditions. Such plants as are best fitted to live under certain conditions are the ones which will survive.

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 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 all parts of the plant. If the plant has floating leaves, as in the pond lily, the stomata are all in the upper side of the leaf.

Plants living in water have not only loose and spongy tissues, but 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 material used in food-making 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 environment are said to be hydrophytic, and such plant is said to be a hydrophyte.

Xerophytes. If we examine plants growing in dry or desert conditions, as cactus, sagebrush, 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.

The stem may be thickened so as to store water; a covering of hairs or some other material may be present and lessen loss of

moisture by evaporation. The conditions of extreme dryness under which such plants live is called xerophytic, and such plants are known as xerophytes. 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 HUNT. ES. BIO.- -11

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,