SEASONS IN SOUTHERN ARIZONA 



193 



The records at the Desert Laboratory, show that the 

 Anemone of this region and the few early blooming species 

 that appear with it as described below, require about a thous- 

 and hours exposure to temperatures above the freezing point 

 to enable them to open flowers receiving in this time (in 

 1907) 2 15 14 hour-degree-Fahrenheit units of heat exposure. 

 During this time the thermometer may drop below freezing 



Fig. 2. — Cassia covesii on Tumamoc Hill. 



point for a total amount of 8 to 10 hours. It is interesting to 

 note in this connection that Draba vernci one of the earliest 

 spring flowering plants of New York opened flowers after 

 974 hours exposure to temperatures above freezing point, 

 recei\-ing in that time 4110 hour-degree-Fahrenheit units of 

 heat (originally calculated in hour-degree centigrade, but 

 transposed here for comparison), and being subject to a 

 much larger number of hours exposure to freezing tempera- 



