NOTE. In case a food which is being tested may possibly contain a small amount of fat, the food should be pulverized, shaken in a test tube with ether or benzine (to dissolve out the fat), and allowed to stand for 24 hours. The clear liquid should then be poured upon paper, and the ether or benzine allowed to evaporate.

Caution. Never handle ether or benzine near a flame or hot stove, since the vapor of these substances is very inflammable.

27. To test foods for mineral matter. Laboratory Study No. 12. Demonstration.

Materials: Salt, corn starch, grape sugar, piece of boiled egg, butter or fat, water.

1. Place a piece of salt half the size of a pea on an old cooking spoon and heat over as hot a flame as possible. Does the salt burn? Does it disappear?

2. In the same manner try the effect of heat on the other food substances (starch, grape sugar, white of egg, fat, water). Do any of these substances burn or disappear? 3. From the preceding experiments state how you can determine whether or not a food contains mineral matter. 4. (Optional home work.) Test at home in the same manner as described in 1 above a match stick and a leaf. Do these plant materials contain mineral matters? How do you know?

28. To test foods for water. Home work.

Laboratory Study No. 13.

1. Warm a little water on a spoon and place over it a dry, cool, tumbler. What do you see on the inside of the glass? How, then, can you tell whether or not water is found in a given food?

2. (Optional.) Test as many foods as you can by warming in turn a small quantity of each in a spoon (without letting the food burn), and holding over the spoon a dry, cool tumbler. What do you learn as to the presence of water in foods?

3. (Optional Demonstration.) To determine the amount of water in potatoes:

a. Remove a thin layer of peel from a potato, weigh the potato, and lay it aside in a warm dry place (protected from mice). Weigh each day, and fill out in your note-book for each day the first, third, and fifth columns in a table like the following:

First day
Second day

Third day

Fourth day

etc.

WEIGHT OF POTATO LOSS OF ORIGINAL WT. PER CENT OF Loss

b. Secure a second potato about the same size as the first, weigh it each day, and place it beside the potato that was peeled. Record results and percentages for each day in second, fourth, and sixth columns of preceding table.

c. Which of the two potatoes decreases in weight the more rapidly? Almost all the loss of weight is due to the evaporation of water; what do you infer, therefore, as to one use of the peel?

d. Continue to weigh the potato without the peel at intervals until there is no further loss. What percentage of potato is water? When potatoes are bought at $3 per barrel, how much of this sum is paid for water?

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.

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.

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 diameter 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

CARA LANG

II-18

strating the relation of carbon dioxid to starch manufacture.

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