EFFECT OF TEMPERATURE ON GROWTH. 75 1 



When De Candolle's inferior limits are below 5° C, they are most probably correct ; 

 his superior limits and temperatures of most rapid growth are, on the other hand, for 

 the most part certainly too low. 



Those figures deserve a more careful study which give the lengths attained by roots 

 in the same periods of time at different temperatures, and express therefore the rate of 

 the growth of the roots of seedlings at different constant temperatures. These numbers 

 increase from the inferior limit to the temperature of most rapid growth, and fall again 

 from it to the superior limit. 



In Zea Mais, for example, I found — 



Temperature. Length attained by the root. 



in 2 X 48 hours 17*1° C. 2*5 mm. 



48 

 48 

 48 

 48 

 48 



Temperature. 



i4-i°G. 



i8-o 

 23-5 

 26-6 

 28-5 

 30-2 

 33'5 

 36-5 



Temperature. 

 21*6 



27-4 

 30*6 



33*9 



37*2 



The importance of maintaining a constant temperature during each experiment for 

 the determination of these cardinal points is especially evident from the fact observed by 

 Koppen, that the same part of a plant grows with very different degrees of rapidity even 

 though the mean temperature be the same ; if, for example, in one case the mean 

 remains nearly constant, while in the other case it varies repeatedly above or below 

 the mean. It is obvious therefore that if the mean tem.perature is that of most rapid 

 growth, every oscillation either upwards or downwards must retard growth. Koppen 

 shows however in addition (/. c. p. 1 7 et seq.) that growth is retarded by considerable 

 oscillations even below this most favourable temperature. He found, for example, that 

 after a seed of Pisum satiinim had germinated for 144 hours at a constant temperature of 

 15-1° C, the root had attained a length of no mm. ; when the temperature was variable, 

 while the earth had twice been heated to 20° C. but had fallen between times to 15° C, 

 the mean being 16° C, the roots grew only to 88 mm.; when the temperature varied 

 between 15-0° and 30*0°, the mean being i8*o°, the length attained by the roots was 

 only 56 mm. Although therefore the calculated mean temperatures were higher than 

 115° C, the growth was retarded, and the more so the greater the oscillations. 



The following table of the lengths attained by the roots in ninety-six hours in each 

 case is taken from a copious list of Koppen's. 



