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IS STARCH PRESENT IN PLANT TISSUES? (VII-1)
The problem. A very common food substance in plants is starch. It is probably known to most of us that there is no source of starch from which most plants can secure it. It would seem probable, therefore, that the plant must make it from such raw materials as it is able to secure. As a first step in investigating the origin of starch and other food materials in plants we need to know of what these foods are composed and whether they are simple substances or compounds.
What to use. Some commercial starch, a piece of fresh white potato, a white-spotted or partially green leaf (as some kinds of geraniums), a dilute solution of iodine (tincture of iodine), and glass plates.
What to do. 1. Place some commercial starch upon a glass plate. Put a drop of the iodine upon the starch and observe the color changes of the starch.
2. Do the same with a thin slice of potato.
3. Place several drops of iodine upon one of the light spots of a leaf and after five or ten minutes remove the iodine and see if the color indicates that there is starch in the leaf.
Questions. What is the color of pure starch when treated with the iodine solution ? Do the fresh potato and commercial starch show the same color when so treated ? Does the leaf? Do you note any color changes in the green part of the leaf? What inferences do you make, and why?
Suggestions for report. Describe the effects of the iodine solution upon starch, and the use of this as a means of finding whether starch is present in plant tissues and in food materials.
Reference work. Read sections 80 to 85.
Optional problems. Apply the above test to other foods. What plants furnish most of the world's supply of starch?
RELATION OF WATER AND CARBON DIOXIDE TO
COMPOSITION OF STARCH (VII-2)
The problem. Since starch is one of the most important foods for men and animals it is worth while to learn something about its composition. In order to approach this problem we need first to be able to answer the questions : Is starch a simple substance or a compound substance ? If it is a compound, of what is it composed ?
What to use. A dry test tube, a small amount of dry starch, and a flame from gas or alcohol.
What to do. 1. Heat the test tube to make sure that it is quite dry. Pour into it enough starch to cover the bottom of the tube. Then, while carefully observing what occurs, heat the tube and starch slowly until the starch is changed so that none of it has the white color which it had when the experiment began.
Questions. What are the drops which appear upon the inner walls of the test tube ? Where do they come from ? Remove some of the black material from the bottom of the test tube and try to determine what it is. In what way does it differ from starch?
Suggestions for report. Describe what occurs when starch is heated in a dry test tube.
Reference work. Read sections 85 to 90. Make a list of the uses to which starch is put in your home and community. See if you can find printed articles which describe the commercial preparation of starch.
Optional problems. Pulverize and dry some starch from a white potato and repeat this experiment with this starch. Are the results like those previously observed ? When the potato is pulverized does the result include anything besides starch? Can you determine whether the heated starch gives off carbon dioxide ?
RELATION OF CHLOROPHYLL TO THE REST OF
THE LEAF (VII-3)
The problem. Some leaves are white-spotted, some clear green, some brown, or of other colors. In the starch tests no very satisfactory results could be seen on the parts of the leaf which were green. Can this green material known as "chlorophyll” be removed from the leaf ?
What to use. Some fresh green leaves (such as those of nasturtium or geranium), two wide-mouth bottles and stoppers to fit, alcohol, boiling water, and iodine solution.
What to do. 1. Dip a few leaves in boiling water, then place them in a bottle of alcohol. Insert the stopper in the bottle.
2. Place others in another bottle of alcohol without having dipped them in boiling water. Insert the stopper. Allow both bottles to stand for two or three hours to see whether the chlorophyll behaves in the same way in both.
3. After twenty minutes remove one of the leaves which was dipped in boiling water and, after rinsing in cold water, note its color. Treat it with iodine to see if it contains starch.
Questions. What happened to the chlorophyll in both sets of leaves ? What differences were there between the two cases ? Is the chlorophyll always a part of the leaf? Does absence of chlorophyll make it possible to determine presence of starch by use of the iodine test?
Suggestions for report. Describe how to remove chlorophyll from a leaf. Tell what chlorophyll is, and how dipping a leaf in hot water affects the removal of chlorophyll by alcohol.
Reference work. Read sections 87, 92, 93, and 94.
Optional problems. Try the tests given above on leaves which have been in darkness for at least ten hours.