THE THERMOMETRIC MINIMUM 137 



plotting of the photosynthetic curve under variation of 

 temperature. 



Another necessary adjustment is the maintenance of 

 constancy of volume of the water in the Bubbler, at the 

 level of the mark m, to which it was adjusted at the begin- 

 ning of the experiment. The expansion of the water during 

 rise of temperature raises this level to a small extent ; the 

 level can, however, be easily readjusted for each series of 

 observations by the manipulation of the adjusting funnel f. 



The Thermometric Minimum 



The critical temperature for arrest of activity is very 

 well denned. Thus, in an experiment with a particular 

 specimen, the critical point of arrest was found to be 9 -5. 

 A rise of temperature of half a degree initiated photosyn- 

 thesis, while a fall of half a degree arrested it. In spring 

 the critical point is a little higher, about n ° C. or so. The 

 plant becomes accommodated to the lower temperature in 

 winter, and the critical point is lowered by a degree and 

 a half. It is interesting to compare the above with the 

 critical point of arrest of the pulsation of the leaflets of 

 the tropical plant Desmodium gyrans. In the summer the 

 point of arrest of pulsation is about 17 , but in winter it is 

 as low as n° C. 



This gradual accommodation to a lower temperature is 

 illustrated in a very interesting manner in the following 

 experiment. A spring-specimen of Hydrilla was subjected 

 to repeated lowering of temperature to the point of arrest. 

 On the first occasion the stoppage of activity occurred at 

 12 C, on the second at n°-5, on the third at 9°-5, and on 

 the fourth at 9 C. ; the fifth repetition did not produce any 

 further lowering of the point of arrest. 



I had found a similar accommodation to lower tempera- 

 ture in the ascent of sap. The power of suction was arrested 

 in tropical specimens of Impatiens when the temperature 

 of the water supplied to the cut end of the stem was 15 C. 



