INFLUENCE OF TEMPERATURE 193 



He finds that at sub-optimal temperatures, 17-3° to 21-6° C, 

 the relative growth rate of the leaf is controlled by light 

 intensity not by temperature. At this sub-optimal range, 

 temperature affects leaf growth by its action on the rate of 

 development in the apical bud, thus controlling the rate of 

 unfolding of the first foliage leaf. In other words, the delay 

 in the development of the first foliage leaf is determined by 

 the temperature coefficient of developmental processes ; with 

 higher temperatures the development of the first foliage leaf 

 is progressively delayed owing to the derangement of develop- 

 mental processes leading to a fall in the temperature coefficient. 

 This is illustrated by the following figures : — 



At sub-optimal temperatures (17-3° to 21-6° C.) . [ ~ 10 ~ 



At supra-optimal temperatures (29-1° to 32-4° C.) . !^ 10 ~ 



This retardation in the apical development at supra-normal 

 temperatures is partly responsible for the reduction in the 

 relative growth rate of the leaf, which also is retarded by a 

 redistribution of material, more growth being made by the 

 stem. These higher temperatures thus introduce a time-factor. 

 At all temperatures, with the light intensity used, a fall in 

 the relative growth rate of the leaf occurs. A second time- 

 factor is thus operating due to the low light intensity, for it 

 disappears on increasing the amount of light. There would 

 thus seem to be a minimum light intensity below which normal 

 growth cannot proceed, and unless this degree of light intensity 

 be maintained, a time-factor appears which continually re- 

 duces the relative rate of growth of the leaf surface. This 

 detrimental effect is not due to a derangement of the corre- 

 lation between stem and leaf growth, nor does it appear to be 

 due to incipient starvation following the low rate of carbon 

 assimilation. The detrimental effect of a low light intensity 

 possibly indicates an effect on the root system, for, in the barley, 

 there is a close relation between leaf growth and root growth 

 which, possibly, is reciprocal, the leaf supplying carbohydrates, 

 which are of great importance in determining root growth, and 

 the roots supplying the nitrogen requisite for the growth and 



VOL. II. — 13 



