lo HORTICULTURAL SOCIETY OF NEW YORK. 



ity clue to the faulty method of calculating such exposures. The 

 performances of an engine are not to be calculated by the total 

 averages of the steam pressure during its working days, but may 

 be quite exactly determined by multiplying the pressure by the 

 number of hours during which pressure was kept up and used. 



A similar relation holds with regard to the use and effect 

 of radiant energy in the plant, and although any method of es- 

 timation must be more or less arbitrary, yet it seems possible to 

 select one which will be capable of wide application and corre- 

 sponding value. In the evolution of such a method for plants 

 in the temperate zone it seems less artificial to begin the calcu- 

 lation of the heat exposure with the winter solstice instead of 

 January ist, and as has been done by several writers, or if eco- 

 nomic plants are under consideration, take the date of planting 

 as a starting point. It also seems most convenient to use the 

 temperature of the freezing point of water as a base line for the 

 thermometry of the plant. 



The application of the method then simply 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, under 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 pla- 

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

 saccharimim were mature and ready for fertilization on March 

 26, 1901, in the New York Botanical Garden, after iioo hours' 

 exposure to temperature above zero with a totality of 3109 

 hour-centigrade units : Draba vcrna attained the same stage 

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

 exposure. 



Now, it is by no means to be assumed that the above data 

 represent the fixed and invariable constant heat exposure of the 

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



