TEMPERATURE AND CROP PLANTS 



135 



mum tcmiierature for germination, or for any other activity, varies 

 greatl\- with different species of i)lants. A somewhat more satis- 

 factory method is to use as the minimum effective temi)erature the 

 mean daily temjjerature at planting time since the two may be 

 shown to approximately correspond. Farmers ordinarily plant each 

 crop at about the same time each year without much reference to any 

 thermometer, but they have, quite unknowingly, hit upon dates 

 that correspond to the minimum temperatures of which we are 

 speaking. Therefore, by noting the average time of planting each 

 crop for a long series of years it is readily possible to determine the 

 mean daily temperature at planting time, and this has been done for 

 a number of crops. 



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Fig. 61.— a "cemetery" forest of Engleman spruce. Killed by fire. 



By using the above method it has been found that the planting 

 of spring wheat begins when the temperature has risen to about 

 37° F., spring oats are planted when the temperature is at 43° F., 

 early potatoes at 45° F., corn at 55° F. and cotton at 62° F. The 

 temperature for the planting of each crop is fairly uniform wherever 

 the crop is grown. For example, early potatoes are planted in some 

 of the southern states as early as the middle of February while in 

 some of the northern states planting does not begin until about 

 j\Iay 1, but in each locality the planting begins when the mean 

 daily temperature has risen to about 45° F. 



The thermal constants have not been actually worked out for 

 many plants but they could readily be computed from data that 



