3. Potato sections, like those of all parts of living plants, are composed of a large number of living cells, each one inclosed by a cell membrane (Fig. 9).

Call to mind what you learned in 45, and state why the cells become more flabby in one solution and more rigid in the other.

47. Will starch pass through a membrane (cell-walls)?Laboratory Study No. 25.

Thistle tube No. 3.- Put into a third thistle tube a mixture of starch and water, cover the bulb with a membrane, and invert in a bottle of water, as already directed for the first thistle tube. See that the level of the liquid is the same in all of the experiments. 1. In what respects does the preparation of thistle tube No 3 resemble that of No. 1? How do the two experiments differ?

2. At the end of a few hours test the liquid in bottle No. 3 by removing a sample to a test tube (as already directed in 45), and adding iodine solution.

a. Is starch present? How do you know?

b. What is your conclusion as to the possibility of starch passing through a membrane?

3. What have these experiments in osmosis taught you as to one difference between starch and grape sugar?

48. Definitions and applications. -The experiments we have been performing have most important relations to the study of all living plants and animals. We may give the following as a definition of the process we are considering: Osmosis is the interchange of liquids of different density that are separated by a plant or an animal membrane (cell-walls). In the process of osmosis the greater flow is always from the less dense liquid to the more dense.

We shall constantly refer to this principle of osmosis, and we shall find that it explains in large measure the absorption of soil water by roots, the transfer of sap from one part of a

plant to another, as well as the processes by which the blood of animals obtains and gives off food to various cells of the body.

By the preceding experiments we have proved that there are two classes of food substances. One kind (including water and grape sugar) will readily pass through a membrane by osmosis; the other kind (represented by starch) will not. In our study of cells we learned that the protoplasm or living substance is inclosed by a cell-wall which separates one cell from another. Now if cells are to make use of the food materials manufactured in other parts of the plant, each food substance must be in such a form that it can pass through these cell-membranes. It is evident that water and grape sugar can do this. We find, however, large quantities of starch stored in cells (Fig. 9). Hence, to be available for use in other cells, some change must be made in this food substance before it can be transferred from cell to cell. We shall now show by experiment what this change is.

49. How starch is made ready to pass through cell-walls. Laboratory Study No. 26.

Into each of two test tubes put a small amount of starch (arrowroot starch if it can be obtained), add some water, shake, and boil. To the starch mixture in one test tube add some diastase, equal in amount to one-half the size of a pea. (Diastase is a chemical substance produced or secreted by the protoplasm of plant cells.) Put the two test tubes side by side in a warm place for 5 minutes if arrowroot starch, 24 hours if corn starch is used, then test a small amount of the mixture in each test tube by adding a few drops of iodine.

1. Describe in your own words what has been done. 2. In which test tube do you find starch present? 3. Now test with Fehling's solution a small quantity of each mixture. In which tube do you find grape sugar?

4. What do you conclude, therefore, as to the effect of dias

tase on starch?

5. Why is this change necessary if starch is to be used by plants?

50. To prove that starch is made soluble in growing plants. - Laboratory Study No. 27.

1. Pound two or three corn grains into a powder and put some of this corn meal into a test tube, add water, and boil. To one-half of the mixture add iodine, and to the other half, Fehling's solution, and boil. Give a careful description of the experiment and state your observations and conclusions.

2. Secure some germinating corn grains, cut them into small pieces, and test some of them with Fehling's solution as in 1 above. Describe the experiment, stating your observations and conclusions.

3. The change in starch that you have described is known as digestion. What reason have you for believing that starch is made soluble when corn grains germinate? This change in starch is known as digestion.

51. Definition of digestion. We may define digestion as the chemical change whereby insoluble food substances are made ready to pass through cell-walls or made ready to be used in cells. Let us now by experiment determine whether or not protein needs digestion.

52. Will protein pass through a membrane (cell-walls)? Laboratory Study No. 28.

Thistle tube No. 4. Secure some white of egg, cut it with scissors and mix it with water. (White of egg, we found, contains a large amount of protein.) Prepare the fourth thistle tube in the same way as directed for thistle tube No. 1, only using white of egg and water instead of grape sugar. See that the level of the liquid is the same as in thistle tube No. 2.

1. In what respects does the preparation of thistle tube No. 4 resemble that of thistle tube No. 1? How do the two experiments differ?

2. Allow the experiment to stand for several hours, and then remove with a glass tube a sample of the liquid in

bottle No. 4, and test it by adding nitric acid and boiling. Is protein present? How do you know?

3. Do you conclude, therefore, that protein will or will not pass through a membrane?

53. Digestive ferments. We have stated that protoplasm secretes a substance called diastase, and have shown that this diastase will change insoluble starch to soluble grape sugar, which will pass from one cell to another by the process of osmosis, or be ready for use in the cells. Diastase is a substance known as a digestive ferment. Now protoplasm produces other digestive ferments, some of which will change proteins to soluble substances that will readily pass through cell-walls by the process of osmosis, and be in such a condition that it can be used by protoplasm.

Fats, also, like starch and protein, are insoluble and cannot, therefore, pass by osmosis through cell walls. To make these food substances available for use they must also be changed by the plant cells into such forms that they may be readily transferred from one part of the plant to another. These changes are caused by other chemical ferments produced by protoplasm.

CHAPTER V

ADAPTATIONS OF THE NUTRITIVE ORGANS OF PLANTS

54. The nutritive organs of plants. From our study of food manufacture (29-34) we learned that the plant foods are produced in green leaves. Before this process of food manufacture can go on, however, the cells in the leaf must be supplied with raw materials from the air and from the soil. Since the roots, stems, and leaves are all concerned in food making, these organs are known as the nutritive organs of plants. Each of these organs has several functions; we shall now learn what some of these functions are, and how the nutritive organs are adapted for the work they do.

I. THE STRUCTURE AND ADAPTATIONS OF ROOTS

55. The structure of roots.

A. Gross structure of roots.

Laboratory Study No. 29.

Select the largest roots of a well-developed seedling or the roots of common weeds. By means of your thumb and finger nail gently scrape off the outer layers from a piece of one of these roots. When no more of the material can be easily removed by this method, pick to pieces the central part of the root which is left. The outer layer you have removed is largely composed of the cells of the cortex, and the central part that has been exposed is called the central cylinder.

1. Tell what you have done.

2. Which is composed of the tougher and harder material, the cortex or the central cylinder?