212 General Botany 



most plants grow best under certain rather fixed temperature 

 conditions. For tropical plants, air temperatures above 90° F. 

 are most favorable. Temperate plants develop best at between 

 60° and 90° F. Arctic and alpine plants grow at temperatures 

 but little above the freezing point. 



The time during which the temperature remains above the 

 freezing point is the growing season. In the tropics this extends 

 throughout the year. In arctic and alpine regions it may be 

 reduced to 2 or 3 months. The temperature of the air and the 

 length of the growing season are important factors in determining 

 the amount of food a plant may manufacture, and consequently 

 the amount of growth. Rice and peanuts, for example, require 

 high temperatures for their best growth, while cotton must have 

 a long season in which to mature its seeds. None of these crops, 

 consequently, is profitable north of Tennessee. 



Air temperatures influenced by air drainage. Cold air is 

 heavier than warm air ; consequently it accumulates in low 

 grounds and reduces the temperature there. In low places frost 

 occurs later in the spring and earher in the autumn than on hills. 

 Crop plants like beans, that are easily injured by frost, can be 

 planted earher and grown later on uplands. Peach orchards are 

 more profitable on uplands than in valley bottoms, because on the 

 uplands they are more likely to escape late spring frosts. 



Soil temperatures also are important. Dark-colored soils are 

 warmer than hght-colored soils of the same texture, because they 

 absorb the sun's rays more readily. Well-drained soils are 

 warmer than wet soils, (i) because less heat is required to raise 

 their temperatures, and (2) because the temperature of a wet 

 soil is lowered by the constant evaporation of water. The most 

 valuable farm lands are those with dark-colored, well-drained 

 soils. On north slopes, soils do not warm up so rapidly in the 

 spring, and plants growing there start their growth later than do 

 those on the south slopes of the same hills. Peach growers prefer 

 not only uplands but north slopes. 



