146 Science of Plant Life 



fall is scanty during the time of highest temperatures, plants 

 are hindered in their growth, and only xerophytes may be 

 able to withstand the conditions. 



The temf)erature factor. As one goes north or south from 

 the equator, the temperatures of the soil and air decrease. 

 Increasing altitude in mountains brings about the same effects. 

 Temperature directly influences the rate of all plant pro- 

 cesses, and most plants grow best under certain rather fixed 

 temperature conditions. For tropical plants, air temperatures 

 above 90 degrees F. are most favorable. Temperate plants de- 

 velop best at between 60 and 90 degrees F. Arctic and alpine 

 plants grow at temperatures but little above the freezing point. 



Air temperatures are greatly influenced by air drainage. 

 Cold air is heavier than warm air ; consequently it accumulates 

 in low grounds and reduces the temperature there. Frost 

 occurs later in the spring and earlier in the autumn in low 

 places than on hiUs. Crop plants like beans, that are easily 

 injured by frost, can be planted earlier 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. 



The time during which the temperature remains above the 

 freezing point is the growing season. In the tropics this ex- 

 tends 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 determine the amount 

 of food a plant may manufacture, and consequently the 

 amount of growth. 



Soil temperatures also are important. Dark-colored soils 

 are warmer than light-colored soils, because they absorb the 

 sun's rays more readily. Well-drained soils are warmer than 



