RELATION BETWEEN TEMPERATURE AND GROWTH 451 



ascending towards the optimum. On reaching and passing 

 that point, however, it is found that the spot of light, which 

 has hitherto tended to move to the right, now has its move- 

 ment suddenly reversed to the left, thus necessitating a 

 corresponding reversal of the balancing rotation. This 

 turning point is extremely sharp and well defined, and enables 

 us to make an accurate determination of the optimum tem- 

 perature within less than a tenth of a degree. From a 

 previous knowledge that the optimum point lies, say, between 

 35 C. and 36 C, the rise of temperature from 35 C. to 36 C. 

 within the chamber may be adjusted to take place in five 

 minutes, that is to say a rise of one-twentieth of a degree per 

 fifteen seconds. The second observer, watching the delicate 

 thermometer in the plant chamber, calls out at every twentieth 

 of a degree of rise of temperature. The first observer, at the 

 recording drum, notes the temperature of the turning point. 

 It has been said before that the permanent rate of growth 

 for any given temperature is always established in less than 

 fifteen seconds of reaching it. The possible error, owing to 

 this lag, could not therefore exceed one-twentieth of a degree. 



Table showing Circular Readings of Balancing Wheel at 

 Different Temperatures. 



I give above two sets of readings of the balancing wheel, 

 made during two experiments for the determination of the 



