1908.] 



PUBLIC DOCUMENT — No. 33. 



75 



In our work we have used a Coradi rolling sphere planimeter, 

 which is one of the most reliable and accurate styles of planim- 

 eter in the market. 



In seeking to apply the method here outlined to the actual 

 determination of specific physiological constants, the first requi- 

 site was a plant which would pass through its various stages 

 rapidly, so that a number of observations could be made in a 

 comparatively short time ; the phase in question must be one 

 easily reproduced, and several individuals of the same age 

 should be under observation at the same time, in order that the 

 length of time required for the completion of the phase may be 

 noted for a greater number. The germination stage of common 

 curled cress was chosen for observation, as it seemed to satisfy 

 the required conditions. Germination is rapid at ordinary tem- 

 peratures, and is very uniform, and the phase can be studied 

 at all seasons of the year, out of doors or in the greenhouse. 



During the past few months 77 thermograph records have 

 been obtained of this phase, and tabulated for study. In these 

 records the sum temperatures above 32° have varied from 2,714 

 to 4,286, and the time occupied for the completion of the stage 

 from 70 to 210 hours. The problem now, with these figures 

 before us, is to determine at once two unknown quantities : 

 first, the critical temperature ; and second, the constant quan- 

 tity of heat above that temperature required to complete the 

 germination phase in the cress plant. 



The method of making this computation will be readily 



M 



M 



understood from the diagram (Fig. 2). The figure M repre- 

 sents one thermograph record for one experiment, and the figure 

 N another record for another experiment. The lines c f and 

 C F represent the 32° or base line. The irregular lines a d 

 and A D represent the temperature trace. The lines b e and 

 B F represent the critical temperature. Temperatures below 



