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IV. MANUFACTURE OF THE FOOD SUBSTANCES BY PLANTS

29. Is starch present in the green leaves of a plant that has been exposed to sunlight? - Laboratory Study No. 14.

Take several leaves from a vigorous plant (e.g. geranium, hydrangea) which has been exposed to bright sunlight for a number of hours. Boil them a few moments in a large test tube or flask of water; pour off the water, add alcohol, and boil carefully over a piece of wire gauze or asbestos until all the green coloring matter has been removed. Rinse the leaves in water, add iodine solution, and spread the leaves on saucers, or in Petri dishes.

1. Describe in your own words how the experiment was performed.

2. Is starch present in the leaves? How do you know? 3. Why was it necessary to remove the green coloring

matter from the leaves before testing for starch? (If you are in doubt, add some iodine to green leaves.)

4. (Optional.) How may grass stains be removed from clothing?

30. Is starch present in the green leaves of a plant that has been deprived of sunlight?1-Laboratory Study No. 15.

Put a vigorous plant (e.g. fuchsia, squash, sunflower, or bean seedling) in darkness for 48 hours or more. Remove several leaves, and treat them as described in 29 above.

1. State briefly how the preparation of this experiment differs from that in the previous experiment.

2. Give your observation and conclusion.

3. State, therefore, whether sunlight is or is not necessary for the manufacture of starch in green leaves.

31. Is starch present in colorless portions of green leaves? -Laboratory Study No. 16.

1 A most suggestive series of experiments on the formation of starch in green leaves is found in the Botanical Gazette for September, 1909, pp. 224-228, by Sophia Eckerson, of Smith College,

Secure a plant having some portions that are colorless (e.g. striped grass). Expose the plant to sunlight for two or three hours, then remove several leaves and test them in the same manner as described in 29 above.

1. State briefly how the preparation of this experiment differs from that of the two preceding experiments.

2. What is your observation and conclusion as to the presence of the starch in the green and colorless portions? 3. State, therefore, whether green material is or is not necessary for the manufacture of starch.

This green material in leaves is called chlorophyll (from Greek chloros green + phullon = leaf).

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32. Is carbon dioxid necessary for starch manufacture in leaves? Laboratory Study No. 17.

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Secure two vigorous potted plants, two bell-jars large enough to go over the plant and pot, and two trays or other receptacles having a greater di

ameter than that of the bell-jar. Place the plants in darkness for 24 hours at least, so that the leaves may be free from starch (see 30 above). Now test the leaves of both plants to make sure they are free from starch. Into one tray pour a quantity of lime water and into the other tap water. Put the plants on supports of some kind so that the pots will not touch the liquid, and cover

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with the bell-jars. (See Fig. 3.) FIG. 3.-Apparatus for demon

strating the relation of carbon dioxid to starch manufacture.

Be sure that the edges of the belljars are covered with the liquid, so that no air can enter the jars. Place both preparations where the plants can get no sunlight for 24 hours in order to give time for the absorption of carbon dioxid in the jar with the lime water. Place both preparations in strong sunlight for several hours.

1. Describe the preparation of the experiment.

2. Examine the lime water inside the bell-jar. What proof have you that carbon dioxid has been absorbed?

3. Remove a leaf from each of the plants and test for starch. Tell what was done and state your observations. Which leaf, therefore, contains starch?

4. What is your conclusion as to the necessity of carbon dioxid for starch manufacture?

5. What chemical elements that are present in starch might be furnished by the carbon dioxid (CO2)?

6. The other raw material needed by plants for the manufacture of starch is water (H2O). What third chemical element found in starch must be furnished by water? 7. Now name the two raw materials used by plants in the manufacture of starch and state the chemical elements which each can furnish.

33. Manufacture of carbohydrates. The substance first made by the combination of carbon, hydrogen, and oxygen in the leaves is not starch, but a simple carbohydrate which is then made into grape sugar. When the plant manufactures more sugar than it needs for immediate use, the surplus is changed to starch, and this is what we have found stored in the leaves.

34. Manufacture of proteins. We have already learned that proteins contain carbon, hydrogen, oxygen, nitrogen, and usually sulphur and phosphorus (see 21). The plant, therefore, must somehow obtain these elements in order to manufacture proteins. It has been proved that plants manufacture sugar, and this probably supplies the necessary carbon, hydrogen, and oxygen. The nitrogen that is needed is furnished by compounds containing nitrogen such as saltpeter (potassium nitrate, KNO3), and the sulphur and phosphorus are secured from mineral compounds known as sulphates and phosphates. These compounds are derived from soil water.

From these compounds, namely, sugar and the

mineral matters containing nitrogen, sulphur, and phosphorus obtained from the soil, the living plant manufactures protein.

35. Do green plants give off a gas in sunlight? - Laboratory Study No. 18.

Into a glass cylinder containing water fresh from the faucet put a small amount of water plant (Elodea, Spirogyra, or Milfoil), holding it to the bottom of the tall jar by means of a weight if necessary. Stand the cylinder in direct sunlight. 1. Describe the preparation of the experiment.

2. What do you observe coming off from the plant? (These bubbles of gas have been proved to be composed of oxygen.)

36. Do green plants give off a gas when deprived of sunlight? Laboratory Study No. 19.

Place the glass cylinder prepared as directed above in darkness for several hours (or, still better, a second cylinder should be used for comparison).

1. In what respects do Experiments 18 and 19 differ?

2. Do you see any bubbles as long as the cylinder is kept in the dark?

3. Under what condition, therefore, does a green plant give off oxygen?

37. The oxygen supply for animals. We have seen that starch is made of carbon dioxid (CO2) and water (H2O). By repeated experiments biologists have proved that in the process of manufacturing carbohydrates more oxygen is present in the CO2 and H2O than is needed. This is the oxygen we have seen given off by the green water plant in sunlight. Every green plant gives off oxygen into the air when manufacturing carbohydrates. Hence, in this process the carbon dioxid is constantly being taken from the air and a fresh supply of oxygen set free.

CHAPTER III

THE STRUCTURE OF PLANTS

38. The parts of a plant. - Laboratory Study No. 20.

Materials: A well-developed bean plant or other seedling or a weed, for each two pupils; one or more plants with flowers and if possible with fruits for demonstration.

Nearly all the plants with which we are most familiar

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