=Oxidation and Deoxidation.=—The union of oxygen with carbon and other substances, which occurs in fires and in the bodies of animals, is called _oxidation_. The separation of the oxygen from carbon such as occurs in the leaves of plants is called _deoxidation_. _The first process sets energy free, the other process stores it up._ Animals give off carbon dioxide from their lungs or gills, and plants give off oxygen from their leaves. But plants need some energy in growing, so oxidation also occurs in plants, but to a far less extent than in animals. At night, because of the absence of sunlight, no deoxidation is taking place in the plant, but oxidation and growth continue; _so at night the plant actually breathes out some carbon dioxide_. The deepest part of the lungs contains the most carbon dioxide. Why was it necessary to empty the lungs as nearly as possible in the experiment with the candle? Why would first drawing a deep breath interfere with the experiment? Why does closing the draught of a stove, thus shutting off part of the air, lessen the burning? Why does a “firefly” shine brighter at each breath? Why is the pulse and breathing faster in a fever? Very slow in a trance? =The key for understanding any animal= is to find _how it gets food and oxygen_, and how it uses the energy thus obtained to grow, move, avoid its enemies, and get more food. Because it moves, it needs senses to guide it. =The key for understanding a plant= is to find _how it gets food and sunlight_ for its growth. It makes little provision against enemies; its food is in reach, so it needs no senses to guide it. The plant is built on the plan of having the nutritive activities _near the surface_ (_e.g._ absorption by roots; gas exchange in leaves). The animal is built on the plan of having its nutritive activities _on the inside_ (_e.g._ digestion; breathing). =Cell and Protoplasm.=—Both plants and animals are composed of small parts called _cells_. Cells are usually microscopic in size. They have various shapes, as spherical, flat, cylindrical, fibre-like, star-shaped. The living substance of cells is called _protoplasm_. It is a stiff, gluey fluid, _albuminous_ in its nature. Every cell has a denser spot or kernel called a _nucleus_, and in the nucleus is a still smaller speck called a _nucleolus_. Most cells are denser and tougher on the outside, and are said to have a _cell wall_, but many cells are naked, or without a wall. Hence the indispensable part of a cell is not the wall but the nucleus, and a _cell may be defined as a bit of protoplasm containing a nucleus_. This definition includes naked cells as well as cells with walls.