134 HEAT 



such as forest, grassland, or desert. For example, we find forests 

 in every one of the great temperature zones of the earth but the 

 species of trees that make up the forests are different in each zone. 

 The relation of temperature to floral distribution is well expressed 

 by what we may call Schimper's First Law. This states that "The 

 type of the flora in so far as it depends on existing factors is de- 

 termined primarily by heat." 



84. Temperature and Crop Plants.— While we must not make the 

 mistake of over-emphasizing temperature as a controlling factor in 

 the distribution of crop plants, yet it is to be expected that 

 Schimper's First Law applies to specific crop plants to the same 

 extent that it does to uncultivated species except as its action is 

 modified or prevented by man. A definite amount of heat is required 

 to mature each crop and various attempts have been made to find an 

 approximate measure of this amount of heat. There is for each crop 

 plant, as well as for every other kind of plant, a minimum tempera- 

 ture at which activity begins. Any temperatures below this mini- 

 mum are entirely ineffective in advancing the crop toward maturity. 

 Therefore, there must be a considerable number of days during 

 which the temperature rises above the minimum if the plants are to 

 mature. 



A summation of the day-degrees of temperature in excess of the 

 minimum from the date of planting to the date of maturity is known 

 as the "thermal constant" and may be used as an approximate 

 measure of the heat required to mature the crop. For example, 

 if 40° F. be taken as the minimum effective temperature, then any 

 day on which the temperature reached 50° F. would count as ten 

 day-degrees, one on which the temperature reached 54° F. would 

 count as fourteen day-degrees, and so on. The thermal constant 

 would then be found by adding together all of these excess day- 

 degrees for the season during which the crop was developing and 

 maturing. Such thermal constants are of considerable practical 

 value since they enable one to determine whether a given locality is 

 favorable for the maturing of a given crop. If the climate of the 

 locality is such that the necessary number of excess day-degrees to 

 make up the thermal constant for the crop under consideration can- 

 not be expected, then it is useless to attempt to grow such a crop 

 in that locality. 



In deriving the thermal constant the minimum eftective tempera- 

 ture has often been chosen arbitrarily and the same one used for all 

 crop plants. Such a method is obviously unsuitable since the mini- 



