The cell is the unit of growth. It is so with all forms of life—plant or animal, insect or bacterium. In the beginning the start is with a single cell, an egg, if you please. After fertilization has taken place, this single cell enlarges or grows. Many changes now occur, all rather rapidly, until the cell walls become too small, when it breaks apart and forms two cells just like the first used to be. This is known as cell division. As growth increases, the number of cells increases also—until in the end there are millions.
Nature of the Cell.
—The cell is very small. In most cases it cannot be seen with the naked eye. The microscope is necessary for a study of the parts, the nature and the character of the cell.
In the first place the cell is a kind of inclosed sac, in which are found the elements of growth and life. Surrounding the cell is a thin wall known as the cell membrane. In plants this cell wall is composed of cellulose, a woody substance, which is thin and tender in green and growing plants, but hard and woody when the plant is mature.
Within the limits of the cell is the protoplasm, the chief constituent of the cell; locked up in this protoplasm is life, the vital processes that have to do with growth, development, individual existence.
Embedded within the protoplasm is another part known as the nucleus and recognized under the microscope by its density. Around the nucleus is centered the development of new cells or reproduction—for the changes that convert the mother-cell into offspring-cells are first noted in this place.
HOW A CELL DIVIDES
The simple steps in cell division are pictured here. Starting with a single cell, growth and enlargement take place, ending finally in cell division or the production of two individual cells.
So much for plant cells. Is this principle different in animals? For a long time it was thought that plants and animals were different. But upon investigation it was discovered that animals were comprised of cells just as plants. And not only was this discovered to be true, but also that animal cells corresponded in all respects to plant cells. Hence in animals are to be found cells possessing the cell walls formed of a rather thick membrane, the granular protoplasm or yoke, and the nucleus established in the yoke.