192 THE PLANT WORLD 



and is almost entirely absorbed by the soil, thus yielding the 

 greatest efficiency in promoting the growth of plants. At 

 levels between 2,000 and 3,000 feet the summation of tem- 

 perature effects following the northward movement of the 

 sun after the winter solstice soon begins to show, many forms 

 awakening to marked activity in January; a large number 

 bloom in early February, maturing fruit in March and April 

 with the diminution of the rainfall. In all of this zvinter wet 

 season, as it may be termed, a checking action is exerted by 

 the low night temperatures, which drop to 30° and even 20'" 

 F. on many occasions; a few warm days bring a luxuriant 

 crop of low herbaceous annuals almost to bloom, which may 

 then be blighted by the frost in January, and in the general 

 average of conditions it rarely occurs that the more preco- 

 cious forms open flowers before the first week in February, 



It has been found by the author that the feature of tem- 

 perature upon which the progress in development of a plant 

 rests is not the average temperature, but is the amount of 

 heat exposure, calculated by applying the number of hours 

 the temperature has run above the freezing point to the height 

 of the thermometer above this critical point. The starting 

 point of such calculations may begin at the winter solstice 

 or, in the case of specially preserved seeds, at the time of 

 sowing. It seems especially applicable to begin with the 

 winter solstice in the case of the plants active in this season, 

 since they begin to show signs of awakening within a very 

 few days after the sun has begun to move northward. (This 

 method was fully described in the PLANT world for 

 October, 1907.) 



The thermal constant for insects, as a practical concep- 

 tion has been worked out independently by Prof. E. D. San- 

 derson, who uses a method parallel to that previously de- 

 scribed by the author. (See Sanderson, E. D. The relation 

 of temperature to the hibernation of insects. Jour, of Econo- 

 mic Entomology, i :57. 1908.) 



