6 ANIMAL BIOLOGY 



in the plant, but oxidation and growth continue ; so at 

 niglit 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 experi- 

 ment? 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 spheri- 

 cal, 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 nudeolus. 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, 



