This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
A good conception of plant life in its simplest form may be obtained by an examination and study of some of the lower organisms, such as the green scum to be seen on damp walls or the trunks of trees, where the water runs down during rain. If a minute particle of this green matter (Protococcus viridis) is put in a drop of water and placed under a high power of the microscope, it will be seen to consist of numerous tiny green bodies of various sizes, invested by a colourless envelope or cell wall. Each individual constitutes a complete plant. The interior is filled with a particle of granular, jelly-like matter, stained green. This jelly-like substance has been named protoplasm, and, as it is present in all living plants and animals, it is considered the seat of life. It is the essential part of the plant, as we shall see presently. When any of the cells has reached full size, it divides into two equal parts, which become separate individuals, and repeat the history of their parent by feeding, growing, and again dividing. Those who would see this process must needs burn the midnight oil, for one-celled green plants manufacture food from the atmosphere by day in preparation for dividing by night. Rain brings down many of these plants from the roof-gutters of the house, and if a drop of water from the water butt is examined in summer or other suitable time it will be found to contain more or less numerous organisms, some consisting of a particle of green jelly, without the cell wall, and larger ones with an investing wall, but both sizes moving about rapidly. The movement is due to the rapid vibration of two slender thread-like portions of the protoplasm, without colour, and therefore invisible till something is put in the water to bring the organisms to a state of rest. After a time they lose these filaments, and become surrounded by a cell wall, like those on the damp wall. From the damp wall, or from the water in the butt, these lowly plants absorb their food, or rather the raw materials from which they manufacture it. Already we can see that the cell wall can be dispensed with as so much dead matter, while the naked protoplasm is still termed a cell, and is equally an individual plant.