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HOW GASES PASS INTO AND OUT OF A LEAF (VII-4)

The problem. Within green leaves and other green parts of plants carbon dioxide and water are made into foods. The water comes up through the plant from the roots. Carbon dioxide and other gases enter and leave the leaf. What is the structure of the leaf which permits the passage of gases ?

What to use. Some rather thick leaves (those of the life plant (Bryophyllum) and live-forever are good, as are most kinds of lily, iris, and amaryllis), hand lens, compound microscope, sharp knife or razor, glass slides, and cover glasses.

What to do. 1. Peel off a small piece of the "skin" (epidermis) from each surface of the leaf, noting especially its thinness and transparency. Place these thin pieces in a drop of water on a microscope slide, under a cover glass, and examine with a hand lens and with the microscope. (See appendix, p. 182, for suggestions on the use of the microscope.) Find how the different cells of the epidermis are arranged, especially the cells and openings of the " breathing pores,” or stomata.

2. Examine a cross section of a leaf with the microscope.

Questions. Why are stomata often called "breathing pores" ? Is the stomatal opening of the same size in all stomata ? Do you think one of the openings changes in size at different times? Does the opening connect with air spaces within the leaf? Do these openings provide ready passageways for gases to and from the outside air ?

Suggestions for report. Make a diagram of a surface view of leaf epidermis and label the parts. Prepare a diagram of a cross section of the leaf and name the parts.

Reference work. Read sections 86 to 90, and the discussions of leaf structure in elementary texts on botany.

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Optional problems. Allow a piece of a leaf with the epidermis

a removed to lie upon the table for a while and note and explain the effects upon the interior leaf tissues. Make a clay model illustrating the appearance of a leaf in cross section.

OXYGEN PRODUCTION DURING FOOD

MANUFACTURE (VII-5)

The problem. Plants not only absorb carbon dioxide gas but give off a gas when they are manufacturing food. This is often noted when gas bubbles are forming about water plants in the sunlight. Can this gas be collected and examined ? If various agencies are constantly using oxygen from the air and adding carbon dioxide to it, why does the oxygen not disappear ?

What to use. An aquarium containing submerged and thrifty water plants (water weed, the pondweeds, water milfoil, and hornwort are common water plants in most parts of the country), test tube, and funnel.

What to do. 1. After the aquarium containing a good supply of water

Fig. 17 plants has stood in the sunlight for fifteen or twenty minutes, gas bubbles may usually be seen to rise from the plants. Collect some of this gas in an inverted test tube (Fig. 17). Test the gas (p. 50) to determine what it is.

2. Place the aquarium in a dark room for a half hour and see if bubbles of this gas are still present.

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Questions. In what sense is it true that a green plant is an oxygen-producing machine? Why does oxygen appear only after a period of exposure of the plants to sunlight ? At what rate is the gas produced ? How does the plant get its carbon dioxide ? Is there oxygen in the water ?

Suggestions for report. State how oxygen is released by green plants when they are under the direct rays of sunlight.

Reference work. Read sections 90 to 98.

Optional problems. In drinking troughs for domestic animals where water stands for a long time, small threadlike green plants (pond scums) often are found growing. See if you can find any oxygen bubbles in such places. Do land plants produce oxygen as water plants do? Of what significance is it to other living things to have green plants use carbon dioxide and release oxygen? Do green plants ever use oxygen and release carbon dioxide ?

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