from the axils of the true, very tiny, scalelike leaves. The spines noted in the cactus are examples of reduced leaves.

LEAVES AS INSECT TRAPS. Most curious of all are the modifications of the leaf into insect traps. It frequently happens that the habitat of a plant will not furnish the raw food materials necessary to form proteid food and to build protoplasm. Nitrogen is the lacking element. The plant has become adapted to these conditions and obtains nitrogenous food from the bodies of insects which it catches. Examples of insect traps are the common bladderwort (Utricularia), the Venus's flytrap (Dionaea muscipula), the sundew (Drosera rotundifolia), and certain of the pitcher plants.

BLADDERWORT.-The simplest contrivance for the taking of animal food by the leaf is seen in the bladderwort. Here certain of the leaves are modified into little bladders provided with trapdoors which open inwards. Small water-swimming crustaceans (as water fleas, etc.) push their way into the trap and there die, perhaps of starvation. Bacteria, causing decay, soon break down their bodies into soluble substances, the nitrogenous portion of which is absorbed by the inner surface of the bladders and used by the plant as food.

VENUS'S FLYTRAP. In the Venus's flytrap, a curious plant found in our Southern states, the apex of the leaf is peculiarly modified to form an insect

trap. Each margin of the leaf is provided with a row of hairs; there are also three central hairs on each side of the midrib. The hairs are sensitive to a stimulus from without. The blade is so constructed that the slightest stimulus causes a closing of the leaf along the midrib. The surface of the leaf is provided with many tiny glands, which pour out a fluid capable of digesting proteid food. Thus an insect, caught between the halves of the leaf blade, is held there and slowly digested.

SUNDEW. In the sundew the leaves are covered with long glandular hairs, each of which is extremely sensitive to the stimulus of any nitrogenous substance. These hairs exude a clear, sticky fluid which first renders more difficult the escape of the insect caught in

Leaf of sundew closing over captured insect. the hairs, and then digests the nitrogenous parts of the insect thus caught. Charles Darwin, in a series of experiments, found that these hairs do not respond to the stimulus of falling raindrops, but that a bit of hair weighing only of a grain is enough to cause the slight bending of the hairs.

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PITCHER PLANTS. The common pitcher plant has an urn-shaped leaf which is modified to hold water. Many small flies and other insects find their way into the pitcher and are eventually drowned in the cup. Whether the plant actually makes use of the food thus obtained is a matter unsettled. In a tropical form, called Nepenthes, the petiole of the leaf forms the pitcher, the blade of the leaf forming a kind of lid. In the fullgrown plants this lid stands open, perhaps as an attraction to insects. Honey glands

on the pitcher lead the insect to its destruction. The insect slips into the fluid in the pitcher, is digested, and the proteid portion absorbed.

Leaves as Food.

Some leaves are used directly by man for food. Examples are cabbage, lettuce, cauliflower, kale, broccoli, and many others. These leaves contain (with a large percentage of water) gluten (a proteid), starch, oil, and mineral matter. These foods, properly admixed with certain fleshy foods, are of great importance in giving a balance to diet.

Economic Use of Leaves. The practical use of green plants to man is very great. Plants give off oxygen in the sunlight and use carbon dioxide, which is given off by animals in the breath. Thus parks containing green trees are truly the breathing places of the city.

Another very important use to man is seen in the fact that leaves, falling to the ground, help to form a rich covering of humus, which acts as a coat to hold in moisture. The forests are our greatest source of water supply. The cutting away of the forest always means a depletion of the reserve water stored in soil, with consequent floods and droughts in alternation.

REFERENCE BOOKS

FOR THE PUPIL

Andrews, Botany All the Year Round, pages 46-62.

American Book Company.

Leavitt, Outlines of Botany. American Book Company.

Dana, Plants and their Children, pages 135-185. American Book Company.

FOR THE TEACHER

American Book Company.

American Book Com

Gray, Structural Botany, pages 85-131.

Goodale, Physiological Botany, pages 337-353 and 409-424.

pany.

Darwin, Insectivorous Plants. D. Appleton and Company.
Green. Vegetable Physiology. J. and A. Churchill.

Lubbock, Flowers, Fruits, and Leaves, Last Part. The Macmillan Company.

MacDougal, Practical Text-book of Plant Physiology. Longmans, Green, and Com

pany.

Report of the Division of Forestry, U.S. Department of Agriculture, 1899.

Strasburger, Noll, Schenck, and Schimper. A Text-book of Botany. The Mac

millan Company.

X. ECOLOGY

Simplest Plant Body a Thallus. It has been found by botanists that the plants which are the simplest in body structure are those which live in the water. Sometimes such simple plants are found upon rocks or on the bark of trees.

A red seaweed, an example of a thallus body.

body. All of these diverse shapes under the general name of thallus. have a thalluslike body.

In such plants we can distinguish no root, stem, or leaf. The plant body may even be spherical in outline and consist of but a single cell. Such are the plants which give the green color often found on the bark of trees. Still other plants are threadlike in appearance. Others, as seaweeds, have a ribbon-shaped of plant body are grouped The simplest forms of plants

Adaptation to Environment. This kind of body is of use to a plant which lives in the water as a root to take in water. Plants, as well as animals, are greatly affected by what immediately surrounds them, their environment. It is believed (and we have shown in our experiments) that the environment (conditions of temperature, moisture, soil, etc.) is capable of changing or modifying the structure of plants very greatly. The change which a plant or animal has undergone, that fits it for conditions in which it lives, is called adaptation to environment.

The factors which act on plants and which make up their environment are soil, water, temperature, and light.

The first plants were probably water-loving forms. It seems likely that, as more land appeared on the earth's surface, plants became adapted to changed conditions of life on dry land.

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.