III. PROTOPLASM AND THE CELL

A Living Plant.

If we walk out any afternoon in the fall of

the year, we notice the many forms of plant life that fill the waste

places along the waysides

and make their way into
the cultivated gardens
and fields, driving out
the rightful inhabitants.
We call such plants
weeds. Let us study
some common weed such
as the yellow-blossomed
butter and eggs or the
ubiquitous shepherd's
purse, with the intention
of finding out how such
plants are so well fitted
to live. If we think of the
plant as a mass of living
matter, we at once are
struck with the evident
fact that the living ma-
terial has taken on very
different forms in dif-
ferent parts of the plant.1
The root below the sur-
face of the ground dif-
fers considerably in form
from the stem, which in turn differs in structure from the leaves.

Shepherd's purse; Fl, flowers; F, fruits; S, stem;
L, leaves; R, roots.

1 The living matter of the plants is covered by dead organic matter formed by the activity of the live part. Thus the soft living mass within may be molded into various shapes, as stem, root, or leaves, with the help of the strong and rigid dead parts of the plant.

differs from each other part, and each has a different work or function to perform for the plant. The root holds the plant firmly in the ground and takes in water; the stem holds the leaves up to the light; the leaves, under certain conditions, manufacture food for the plant; the flowers form the fruits; the fruits hold the seeds, which in turn reproduce young plants of the same kind.

Organs. Each part of a plant or animal, having a separate work or function,is known as an organ. Most plants and animals are composed

of organs; hence any living thing, even the simplest single living cell, has come to be called an organism. If we look rather carefully from all sides at the organ called the leaf, we find that the materials of which it is composed do not appear to be everywhere the same. The leaf is much thinner and more delicate in some parts than in others. Holding the flat, expanded blade to the branch is a little stalk, the petiole, which extends into the blade of the leaf as a series of little veins which evidently form a framework for the flat blade somewhat as the sticks of a kite hold the paper in place. In the same manner the veins, if cut crosswise and mounted on a glass slide under the compound microscope,1 show that they are made up of building material which, although microscopic in size, yet differs considerably from other material in the same part of the vein. The smallest units of building material of the plant or animal disclosed by the compound microscope are called cells. The organs of a plant or animal are built of these tiny structures.

Section through the blade of a leaf, as seen under the compound microscope; I, air spaces, which communicate with the outside air.

Tissues. The cells which form certain parts of the veins, the flat blade, or other portions of the plant, are often found in groups 1 For a study of the compound microscope, see Hunter and Valentine, Manual, page 3.

or collections, the cells of which are alike in size and shape Such a collection of cells is called a tissue. Examples of tissues are the cells covering the outside of the human body, the cells which collectively allow of movement, the so-called muscles; the material that forms the framework to which the muscles are attached, the bony tissues; and many others.1

Cells. A cell may be defined as the smallest bit of living matter that can live alone. All plant and animal cells appear to be alike in the fact that every living cell possesses a structure known as the nucleus, which is found within the body of the cell. The nucleus is composed of

living matter like the rest
of the cell, although it
seems to differ in some
chemical way from that
part of the cell surround-
ing it. This is seen when
a plant or animal is placed
in a liquid containing
some dye such as log-
wood. Certain bodies in
the nucleus take up the
stain much more readily
than the rest of the liv-
ing matter of the cell,
taking on a deep black
color. They are
called the chromosomes

thus

(color-bearing bodies).2

Diagram of a cell (after Wilson). The cell protoplasm contains cell food (C.f.); spaces contain liquid cell sap (C.s.); just above the nucleus (N.I.) is a structure called the centrosome (c), which aids in cell division; within the nucleus are chromosomes (N.n.), which form a network; t.n., nucleolus.

1 A very simple tissue that may be studied as an introductory study with the microscope is a thin section of elder or pith, mounted in water or glycerine (dilute) on a glass slide. (See Hunter and Valentine, Manual, page 7.) The study of yeast may be introduced at this point if microscopes are available; at any rate demonstration material showing isolated cells and tissues should here be shown.

2 The chromosomes, which are believed to be always definite in number for every tissue cell, are of much interest to scientists. It is found that each time a cell splits to form two new cells, the chromosomes split in half and an equal number of parts go into the nucleus of each new cell thus formed. These chromosomes are supposed to be the bearers of the qualities which we believe can be handed down from plant to plant and from animal to animal; in other words, the inheritable qualities which make the offspring like its parents.

Protoplasm. The living matter of which all cells are formed is known as protoplasm (from two Greek words meaning first form).

The bulk of the nucleus is filled with a fluid; in some nuclei a body known as a nucleolus is found; it does not seem to be a constant structure. The protoplasm surrounding the nucleus is

called cytoplasm because it makes up the body of the cell. The nucleus plays a very important part in the

life of a cell. Cells grow to a certain size and then split into two new cells. In this process, which is of very great importance in the growth of both plants and animals, the nucleus divides first. The

chromosomes also divide, each splitting lengthwise so that an equal number go to each of the two cells formed from the old cell. Lastly, the cytoplasm separates and two new cells are formed. This process is known as fission. It is the usual method of growth found in the tissues of plants and animals.

The protoplasm in some cells collects into little bodies called plastids. In plant cells the plastids are frequently colored green. This green coloring matter, which is found only in plant cells, is called chlorophyll and green plastids are called chlorophyll bodies. The cytoplasm of a cell contains spaces, which are usually filled with a fluid known as cell sap. These spaces in the cytoplasm are given the name of vacuoles. Frequently non-living materials are found within the cytoplasm of the cell.

The cell is surrounded by a very delicate living structure called the cell membrane. This is so thin that it is impossible to get a microscope of power enough to throw any light on its structure. Outside this membrane a wall is formed by the activity of the protoplasm of the cell. The cell wall is usually much heavier and more conspicuous in the cells of plants. In the cells of the pith it was the wall of cellulose or wood that you saw under the microscope.

STRUCTURE OF PROTOPLASM.- Protoplasm, when viewed under a high magnification of a compound microscope, is a grayish, almost fluid mass, seemingly almost devoid of any structure. A careful observer will, however, find that the material seems to be made of a ground mass of fluid with innumerable granules of various size and form floating in the fluid portion. Other observers believe protoplasm to consist of a fluid groundwork with innumerable tiny threads scattered through it, each thread being more or less firmly united with other threads of the mass. Still other scientists hold that protoplasm has essentially the structure of an emulsion or froth or foam. To them the fine structure resolves itself into a collection of very minute bubbles. Doubtless all of the observers are right in part, for protoplasm doubtless assumes all of the above-mentioned forms in different plants and animals and under different conditions. But we must also bear in mind that when we make observations on protoplasm it may be already dead when we examine it - and therefore undoubtedly greatly changed in structure else we may view it under conditions which are far from the normal conditions under which it usually exists as living matter. Finally, the instrument we call the microscope, although seeming to be nearly perfect, may not always give to our eye an exact representation of what is under its lenses.

Cells of Various Sizes and Shapes. Plant cells and animal cells are of very diverse shapes and sizes. There are cells so large that they can easily be seen with the unaided eye; for example, the root hairs of plants and eggs of some animals. On the other hand, cells may be so minute that in the case of the plant cells we call bacteria, several million could be placed on a dot of this letter i. The forms of cells may be extremely varied in different tissues; they may assume the form of cubes, columns, spheres, flat plates, or may be so irregular that description is impossible. One kind of tissue cell, found in man, has a body so small as to be quite invisible to the naked eye, although it

or

has a prolongation several feet in length. Animal cell, showing netlike

Such are some of the cells of the ner