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than many other plants. Compare the spines of the honey locust, black locust, and barberry. Look for leaf traces and buds on the stem, and decide

from the relative position of the thorns and these structures which of the above-named structures are modified leaves. (Sometimes a spine may be part of a leaf, as the stipule.) Cactus. - In the prickly pear cactus, notice that above the spines are little buds. The position of the bud shows the spine to be a modified leaf. What reason can you give for this modification of the leaf of the cactus? How is the plant body modified to meet the conditions of life in a desert? Note the thickened stem.

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A cactus, showing the leaves modified into spines.

If a cactus is cut open, it will be found to contain a very considerable amount of water. The Indians of the New Mexican desert region, when far from a source of water, sometimes cut off the top of a large cactus, mash up the soft interior of the thickened stem, squeeze out the pulp, and thus obtain several quarts of drinkable water.

PROTECTION BY HAIRS. - In the mullein, one of our hardiest weeds, the leaf is covered with a coating of finely branched hairs. Might such a covering be of use to the leaf? In what ways?

-Sometimes, as in the leaf of the pea, a
for the purpose of climbing.
In this case

part of the leaf is modified
a part of the leaf, called the tendril, becomes especially sensitive to the
stimulus of touch, and upon touching an object coils around it. Almost
any part of the leaf, or indeed the entire leaf, may be modified to become a
tendril. What part of the leaf of the pea here forms the tendril? If
material can be obtained, work out the morphology of modified parts of the
clematis; wild grape; Virginia creeper.

STORAGE OF FOOD OR WATER IN LEAVES. Leaves may be modified for the storage of food or water. Test an onion, which is a collection of thickened leaves closely wrapped to form what is called a bulb, for starch, sugar, and proteid. Squeeze the leaves of the Sedum and notice the water contained in them. The Agave is a desert plant in which the leaves have become greatly thickened as a water and food storage. Make a list of any plants you know, as the cabbage, that store food in the leaves.

REDUCED LEAVES.-Leaves may be reduced to scales or lost altogether. In the asparagus what seem to be tiny leaves are branches which spring


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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 (Dionœa muscipula), the sundew (Drosera rotundifolia), and certain of the pitcher plants.

Bladderwort, showing finely dissected submerged leaves bearing blades which capture animalcula.

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


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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 7840 of a grain is enough to cause the slight bending of the hairs.


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

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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.

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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.

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American Book Company.

Andrews, Botany All the Year Round, pages 46-62.
Leavitt, Outlines of Botany. American Book Company.

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


Gray, Structural Botany, pages 85-131. American Book Company.

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


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-


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.


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. 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

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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.

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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.

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