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?

3. Make a diagram greatly enlarged of a piece of root prepared as directed above. Label cortex, central cylinder, fibers of central cylinder.

B. Root-hairs.

Note to the Teacher.

Root-hairs may be grown for study as follows: Cover the bottom of as many Petri dishes as are needed with a layer of blue blotting paper. Soak the paper with water and lay several grains of soaked barley, oats, or corn upon the bottom of each dish. Put the covered dishes in a warm place for several days. When the root-hairs have developed, wipe the moisture from the inside of the covers, quickly replacing the latter. If Petri dishes are not available, two clean glasses of any convenient size may be used instead. Cover one of the plates with layers of wet blotting paper, put the soaked grains in position, and cover with the second glass, fastening the two together with threads or strings. Stand one end of the preparation thus made in a jar with enough water to reach the lower edge of the blotting paper.

Examine first with the naked eye and then with a hand magnifier the roots of sprouted grains, developed as described above. Notice tiny outgrowths from the sides of the roots; these outgrowths are called root-hairs.

1. Look at the very tip of the root and state whether root hairs are there present or absent.

2. State whether the root-hairs are longest near the tip or in the direction of the grain.

3. Make a drawing much enlarged to show the shape of one of the roots including the root-tip and the various lengths of root-hairs. Label root-tip, roothairs.

C. Microscopical structure of the tip of a root. (Optional.) Examine with the aid of the low power of the compound microscope a root-tip mounted on a slide in drop of water and covered with a cover glass. Make a sketch very much enlarged to show

1. The outline of the root including the tip.

2. A loose mass of cells covering the lower end of the root which make up the root-cap.

3. Label root-tip, cells of the root-cap.

56. The functions of roots.

Laboratory Study No. 30.

A. Roots as organs for holding the plant to the soil.

Secure a vigorously growing plant in a pot (e.g. a rubber plant) or better try the following experiment on a good sized weed in a field. Attach to the stem just above ground level a spring balance. Pull on the balance until the plant shows signs of letting go its hold on the soil, then note the reading in pounds on the scale.

1. In your own words describe what was done.

2. How much force in pounds was exerted on the plant?

3. What important function of roots is shown by this experiment?

B. Roots as organs for absorbing soil-water.

(Before proceeding further with the root study, the osmosis experiments, 44-53, should be performed if they have not already been done.)

Study the diagram of a root-hair in the text-book (Fig. 12) and if possible examine with the low power of the microscope some of the younger (shorter) roothairs. Each root-hair is an elongated part of an outer cell of the root.

1. Draw in your note-book a diagram of a root-hair, labeling cell-wall, thin layer of protoplasm, cellsap, and nucleus.

2. What separates the soil-water from the cell-contents?

3. Recall the characteristics of cellular structure as

given in 42. Now state which is the more dense, the soil-water or the cell-contents.