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 in the plant, but oxidation and growth continue; so at night the plant actually breathes out some carbon dioxid. The deepest part of the lungs contains the most carbon dioxid. 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, fiber-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,