TEMPERATURE AND PLANT DISTRIBUTION 133 



flowers while the ground is still partly covered with snow and con- 

 tinue to flourish even though the temperature goes below freezing 

 each night, while some tropical species carry on their activities at 

 temperatures so high that many plants, if subjected to them, would 

 perish in a very short time. Some plants, such as certain trees, 

 can endure a very wide range of temperature, while others, espe- 

 cially some algse, are active only within a very narrow and very 

 definite range of temperature. 



82. Soil Temperatures.— The temperature of the soil is very im- 

 portant to the plant largely because of its effect upon the rate of 

 absorption of water. Seeds cannot germinate until the soil has been 

 warmed up to a certain temperature, depending upon the species, 

 nor can perennial plants become active in spring until the soil is 

 sufficiently warm so that water can be absorbed readily by the roots. 



Soil temperatures vary greatly in different situations. The soil 

 of a bottomland is usually considerably cooler than that of the ad- 

 jacent upland, particularly if the two are supporting comparable 

 types of vegetation. The soil of a south-facing slope warms up much 

 more rapidly in spring than does a north-facing slope and therefore, 

 dandelions, hepaticas, and other spring flowers bloom earlier on a 

 south-facing slope than elsewhere. 



Soil temperature depends in part on the color of the soil surface. 

 A dark-colored soil absorbs heat and so warms up more rapidly than 

 a light-colored soil which reflects the heat rays. A cold soil can be 

 warmed to a certain extent by spreading over the surface a layer of 

 dark-colored soil. The soil temperature also depends to a certain 

 extent upon the water content, a wet soil being colder than a drier 

 one of similar type, and one result of drainage, though not the most 

 important one, is the increase in temperature through a decrease in 

 water content. 



83. Temperature and Plant Distribution.— The distribution of 

 plants is governed to a certain extent by temperature. Thus, broad 

 belts of vegetation may be recognized between the equator and the 

 poles extending parallel with the equator and corresponding roughly 

 to temperature belts. One also encounters somewhat similar belts 

 in going up a mountain and the corresponding belts are usually 

 somewhat higher on the south side of the mountain than on the north 

 side, because of the difference in temperature. The kind of vege- 

 tation, however, is not so much affected by temperature as is the 

 flora. By the flora of a region we mean the species of plants that 

 occur there, while the kind of vegetation refers to the general type, 



