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 feathers. 



Successive positions, from photographs, showing 

 effect of gravity on a green plant {Impatiens glandii- 

 ligera). — After Pfeffer. 



