LIGHT 



293 



Am. Mus. Natural Hist. 

 Mosses, lichens, and dwarf shrubs are the only vegetation found in the arctic region, 



(tundra zone) . 



and plants at different elevations, the determining factor being 

 largely differences in temperature. Animals normally living in 

 the tropics, if brought to this country, may live, but rarely repro- 

 duce. In such cases all the environmental factors except that of 

 temperature are the same. 



Light. It is easy to pick examples of the effect of light on green 

 plants. For example, we have the turning movements of leaves 

 | and stems, the shape and color of plant leaves, and the presence or 

 ' absence of plants in a given region. But only recently has it been 

 discovered that the flowering of certain plants depends on a lack, 

 : rather than an abundance, of sunlight. Such is the chrysan- 

 i themum, which flowers when the days become shorter. 



Plants and animals are sometimes grouped according to the in- 

 i tensity of light in the environment. Their activity depends upon 

 light, as is seen in the comparative activity of bees on a sunny and 

 : on a dark day, or the activity at night of some nocturnal animals, 

 as the owl or coyote. Green plants are tremendously changed if 

 kept in an environment lacking in light. Compare the sprouts 

 of a potato kept in darkness with one grown in the light. 



Examples of light affecting animals are many. We know that 

 many animals respond negatively to strong light, as owls, bats, 



H. BIO — 20 



