SEASONAL ACTIVITIES OF PLANTS. 225 



The application of the method then simplv entails the cal- 

 culation of the number of hours to which a plant has been ex- 

 posed to temperatures above the freezing point from the winter 

 solstice or other starting point until the stage of development, 

 such as flowering or fruiting, imder consideration has been 

 reached. The time factor is then properly applied to the height 

 of the thermometer above the freezing point during the period 

 mentioned. In actual practice this may be easily done by com- 

 puting the area enclosed by a thermographic tracing of the tem- 

 perature and the base line of the sheet for the period over which 

 the development of a plant is to be studied by means of a plani- 

 meter. It was found by this method that the flowers of Acer 

 saccharinum were mature ?»nd ready for fertilization on March 

 26th, 1901, in the IsTew York Botanical Garden, after 1100 

 hours' exposure to temperature above zero with a totality of 3109 

 hour-centigrade units : Draha veima attained the same stage 

 something earlier in 974 hours, with 1644 hour centigrade-units 

 exposure. 



!N"ow, it is by no means to be assumed that the above data 

 represents the fixed and invariable constant heat exposure of the 

 plants in question, for as has been described previously, the car- 

 dinal points, including the optimum for growth and develop- 

 ment may be altered by other conditions which affect the plant. 

 The variation on which a plant is capable represents its possible 

 geographical range which may be mapped with fair accuracy. 

 Thus the individuals of a species which live nearest the pole have 

 made such accommodations that they are able to accomplish de- 

 velopment with a minimum number of heat units, in a minimum 

 number of hours. As the range of a species is traversed toward 

 the equator or to lower elevations, a place is reached where the 

 heat exposures are at an optimum for the plant, and beyond this, 

 development is retarded until the southern limit of the species is 

 reached. The actual limits are of course determined by the en- 

 tire complex of conditions, of which insolation is also an im- 



