ii DEVELOPMENT AND ENVIRONMENT 29 



by limited defoliation, or by root-pruning, or by cul- 

 tivation in limited quantities of soil, as in flower pots. All 

 these methods give similar results as regards the " sun- 

 shine effect." 



The qualification "limited" is added to the mention of 

 defoliation methods because of the interference with photo- 

 synthesis which follows if too many leaves are removed, 

 or if they are removed from near the terminal bud. 



The positive effect of day temperatures on growth is 

 thus almost non-existent, and the modicum of truth which 



|o. 



Hour 12 

 ending Noon 



Mid- 

 night 



12 

 Noon 



Mid- 

 night 



FIG. 34. THE SUNSHINE EFFECT. 



Growth-rate per hour in hypocotyl of an Assili seedling sown on March 7th, 

 during March 19th to 21st. Compare screened portion with temperature 

 in Fig. 33. 



attaches to " accumulated- temperature " approximations is 

 still further diminished. The only effect of day-tempera- 

 ture is the prejudicial one of " thermotoxy," excepting on 

 the rare occasions when the sky is overcast. Such days 

 are remarkably interesting; usually the clouds take the 

 form of a dust-storm, accompanied by high temperatures, 

 and so long as the temperature does not reach the 

 " thermotoxic " limit, or 35 C., they cause abnormal 

 growths to be recorded, since elongation of the stem is 

 continuous throughout the twenty-four hours. 



In view of the rarity of such weather we are justified 

 in deducing the conclusion that night-temperature is the 

 prime factor in growth during the first half of the growing- 

 season, provided only that no limiting factor other than 



