To this substance is given the name protoplasm. We know now that the living substance or protoplasm is the essential part, while the wall may be missing, so that in such a case there is no resemblance to a cell or box. Biologists now understand a cell to be a bit of protoplasm (cell-body) containing a nucleus (which is a denser portion of the protoplasm). Protoplasm, when examined with the highest powers of the microscope, appears as a colorless, semifluid substance, in which are often seen solid particles or granules, which are probably little masses of food. The nucleus, as already stated, is commonly found near the center of the cell, and is composed of protoplasm denser than the protoplasm of the body of the cell. The appearance and composition of the protoplasm may be well represented by raw white of egg; but in making this comparison one should bear in mind that the white of an egg is not living substance.

Within the cell, too, and occupying some of the space outside the nucleus, especially in plant cells, is cell-sap, which is a lifeless fluid composed of water in which are dissolved the food substances (such as sugar and mineral matters) used by cells in their growth and repair, and in the various kinds of work which they carry on (Fig. 6).

43. Assimilation, growth, and cell division. To make protoplasm the plant must have proteins, water, and additional compounds containing iron, calcium, and several other chemical elements. But only protoplasm has the power to combine these compounds in such a way as to form living matter. Bearing in mind the facts we learned in studying food manufacture (33 and 34), we see that the plant begins with simple substances, water and carbon dioxid, and manufactures a more complex substance, sugar. It uses this and other substances to make a still more complex substance,

protein, and finally ends by making the most complex of all, protoplasm. But, except in rare cases, all plants must have compounds to start with; they cannot make any of these nutrients or protoplasm from chemical elements.

And thus we learn that food materials are gradually changed by protoplasm into living substance like itself. To this process is given the name assimilation

of the process of assimilation the amount of protoplasm of course increases and the cell grows. Were this process to continue indefinitely, cells would become large in size. This, however, does not occur; for when a cell reaches its normal size, the nucleus divides, and the halves separate from each other to form two nuclei. The cell-body now divides into two parts, and cell-walls are formed between the two cells (Fig. 7). Thus are produced two cells, each having its own nucleus, and these in turn assimilate, grow, and divide. In this way the number of cells increases with the growth of the plant.

A, cell before division; B, cell with divided nucleus; C, single cell that has divided into two cells.

CHAPTER IV

OSMOSIS AND DIGESTION

Materials: Four thistle tubes, four wide-mouthed bottles; honey, molasses, or a thick solution of grape sugar; starch (arrowroot if possible), diastase; white of egg, peptone; iodine, Fehling's solution, nitric acid. Procure the intestines of calf or beef, wash them thoroughly inside and out, and inflate them by the aid of a glass tube. Tie at intervals of two or three feet, and allow this animal membrane to dry. Cut off pieces about two inches iong, and slit open

each of the pieces thus obtained. Membrane prepared in this way may be kept in closed bottles for years. If desired, the pieces of membrane may be used at once without drying. Sausage coverings preserved in salt may be thoroughly washed, dried, and used. This membrane is made of cells.

Thistle tube No. 1.- Hold one of the thistle tubes upright, closing the smaller end by pressing on it with the thumb. Into the larger end pour the honey, molasses, or grape sugar solution, which has been sufficiently warmed to pour easily. Half fill the tube and nearly fill the bulb. Moisten one of the pieces of intestine and tie it tightly over the bulb of the thistle tube so that none of the liquid can escape. Wash off any of the liquid from the outside of the membrane, then dry it with a blotter, and hold the thistle tube bulb down for several minutes to make sure that the grape sugar solution does not leak out. Now stand the

FIG. 8.

Apparatus

for thistle tube No. 1 in osmosis experiment.

tube, membrane down, in one of the wide-mouthed bottles and fill it with water up to the neck. Add grape sugar solution to the thistle tube until the level of the water in the bottle and that of the liquid in the thistle tube is the same. Connect a long piece of glass tubing to the upper end of the inverted thistle tube, and support this tube in a vertical position, so that the membrane does not touch the bottom of the bottle (Fig. 8).

Thistle tube No. 2.-Set up a control experiment exactly like No. 1, except that water should be put into the thistle tube as well as in the bottle.

44. Will water pass through a membrane (cell-walls)? Laboratory Study No. 22.

1. Give in your own words a description of the way thistle tube No. 1 was prepared, making a diagram of the apparatus, and labeling level of water in bottle and of grape sugar solution in thistle tube at the beginning of the experiment.

2. At the end of a few hours compare the level of the liquid in thistle tube No. 1 with the level in thistle tube No. 2.

a. How many inches has the grape sugar risen in No. 1? b. Is there a similar rise in the water in thistle tube

No. 2?

c. What must have passed into thistle tube No. 1 to cause the liquid to rise?

d. Through what must this liquid have passed to get into the thistle tube?

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

45. Will grape sugar pass through a membrane (cell-walls)? -Laboratory Study No. 23.

1. At the end of a few hours test the liquid in bottle No. 1 by putting a glass tube to the bottom of the bottle, pressing the thumb over the top of the tube, and removing the sample of liquid thus obtained to a clean test tube; add Fehling's solution and boil.

D

a. Describe what was done.

b. Is grape sugar present now? How do you know? c. What must have happened to produce this result? 2. We have now proved that two different liquids have passed through the membrane.

a. Name these two liquids.

b. Which of these two liquids has passed through the mem-
brane in the greater quantity? How do you know?
c. Which of these two liquids is the thicker or denser?
d. By a great many experiments it has been proved that,
when any two liquids of different density are sep-
arated by a plant or animal membrane, results sim-
ilar to those noted above follow. To this inter-
change of liquids is given the name osmosis. In this
process of osmosis, is the greater flow of liquid from
the less dense to the more dense, or from the more
dense to the less dense?

e. Why did not the water rise in thistle tube No. 2? 3. Do you conclude, therefore, that grape sugar will or will not pass through a membrane?

46. Osmosis in living cells. Laboratory Study No. 24.

Peel a potato and then cut several cross sections about inch in thickness. Allow these sections to stand in the

FIG. 9. Cells from a potato, showing cell

walls, cell-sap,

and

starch grains of differ

ent sizes.

air until they bend readily. Half fill one tumbler with water and a second tumbler with a strong solution of sugar or salt. Place some of the sections in each of the two tumblers and leave them for several hours.

2.

1. Describe the preparation of this
experiment.
Remove a section of potato from
each of the liquids and bend
them. Compare the change that
has taken place in the rigidity or
stiffness of the sections placed

in the strong solution and those in the tap water.