124 GROWTH 



soluble food, and, concurrently, the rapid accumulation of the 

 products of respiratory processes, which also are accelerated by 

 an increased temperature, may have a toxic action. In a few 

 words, up to a certain degree a rise in temperature accelerates 

 physiological actions and here a Van't Hoff curve may be 

 expected : beyond this degree, co-ordination becomes less and 

 less, wherefore metabolic derangement obtains. Priestley and 

 Pearsall,* in their examination of Leitch's results, point out 

 that the growth rate of radicles is dependent on the chemical 

 reactions involved in the merismatic activity of the growing 

 point, and on the hydrolysis of the reserve food in the seed and 

 the translocation of the products to the growing parts. 



The situation therefore is this : the increase in temperature 

 accelerates both growth and hydrolysis, but the merismatic 

 tissue quickly uses up the food immediately available, wherefore 

 a decrease in the rate of growth must ensue for that period of 

 time taken by the food materials in their translocation from the 

 seed to the apical regions of the root. The arrival of this food 

 accounts for the second maximum in Leitch's curves. Finally 

 the growth rate is diminished by the dislocation of the meta- 

 bolic processes. If this contention be correct, a close correla- 

 tion between the length of the root and the time of the incidence 

 of the second maximum should obtain. 



LIGHT. The influence of light on growth is a subject of 

 considerable magnitude, especially when the term growth is used 

 in its general sense : the directive action of light in tropistic 

 and kindred phenomena, its influence in the determination of 

 growth form and the facies of a flora are aspects of the subject 

 outside the scope of the present consideration. 



The action of light on growth is both direct and indirect, 

 and its action is most marked in those members which use 

 light as a source of energy. Thus for the ordinary green 

 plant, increase without light is an impossibility since light is 

 the source of energy for the making of food; in non-green 

 members of plants, in total parasites, and saprophytes, on the 

 other hand, light, for obvious reasons, is not a factor of 

 consequence. It is a laboratory commonplace to find that 

 for the subjects generally used for experimental purposes, light 



* Priestley and Pearsall : " Ann. Bot.," 1922, 36, 239. 



