. Biology; the story of living things. 16 NATURAL HISTORY The distribution of fishes and other organisms in water depends largely on whether these waters are neutral, acid, or alkaline. Brook trout, for example, are usually found in acid and neutral waters, while sunfish, bass, perch, and certain other fish are typically asso- ciated with alkaline waters. Carbon dioxide in the atmosphere is another factor which deter- mines plant distribution, three parts to 10,000 being necessary if plants are to make starch. Oxygen is essential for living things. Certain so-called anaerobic bacteria and a fe
. Biology; the story of living things. 16 NATURAL HISTORY The distribution of fishes and other organisms in water depends largely on whether these waters are neutral, acid, or alkaline. Brook trout, for example, are usually found in acid and neutral waters, while sunfish, bass, perch, and certain other fish are typically asso- ciated with alkaline waters. Carbon dioxide in the atmosphere is another factor which deter- mines plant distribution, three parts to 10,000 being necessary if plants are to make starch. Oxygen is essential for living things. Certain so-called anaerobic bacteria and a few animals appear to be able to live without oxygen. Some insect larvae, worms, and molluscs live a part of the year in deep lakes where little or no free oxygen is present, due to decomposition of the algae. Certainly one factor in the distribution of aquatic animals appears to be the oxygen content of the water. Gravity as a Factor The pull w^e call gravity brings about differences in pressure both of air and of water. Plants and animals must adjust themselves to this factor. In a general way gravity determines the size of organisms. Insects and birds which move about swiftly in the air must be small, otherwise gravity would bring them down. Gravity is important in the growth and orienta- tion of plants. It is a stimulus for the direc- tion taken by the plant body, apparently caus- ing the root to grow downward and the stem to grow upward, while horizontal branches are neutral to the pull of gravity. This same force acts upon sessile or rooted animals, such as hydroids and sponges. Adaptations to offset the force of gravity are seen in the air spaces of floating plants, oil drops in eggs, spines and long hairs on the surfaces of aquatic plants and animals, and the air spaces in bones and other tissues of birds, and in the construction of Successive positions, from photographs, showing effect of gravity on a green plant {Impatiens glandii- ligera). — After Pfeffer.
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