Mar. 1, 1920 Effect of Length of Day on Plant Growth 593 



That temperature is a factor of first importance in influencing and con- 

 trolling plant activities is well understood, and it needs to be considered 

 here only in its relations to the length of day as a factor playing a dom- 

 inant role in the reproductive processes of the plant. It is well known 

 that in accordance with Vant HofF's law the speed of chemical reactions 

 is doubled for each increase of 5° to 10° in temperature; and similarly 

 plant activities and processes such as respiration and growth are acceler- 

 ated by increase in temperature, provided the optimum is not exceeded. 

 Conversely, decrease in temperature may moderately retard the plant's 

 activities, or this effect may increase to more or less complete inhibition. 

 Extremes in either direction, of course, may result in killing the flowering 

 buds or fruits, the vegetative shoots, or the entire plant. It is a matter of 

 common knowledge that low temperature retards the development and 

 the unfolding of flowers. An interesting interrelationship of this action 

 of temperature and that of the seasonal decrease in length of day is seen 

 in the behavior of such trees as the apple, previously referred to. Under 

 the influence of the relatively short days fruit trees of this type might be 

 expected to unfold their flow^ers regularly in the fall instead of in the 

 spring were it not for the interference of low temperatures. The low 

 temperature of winter would seem to have the effect of changing what 

 would otherwise be among the latest flowering plants of the fall into the 

 early flowering ones of spring. 



For the temperate and frigid zones the results of the present investi- 

 gation have made it clear that in some species, at least, distinctions in 

 the time of flowering and fruiting of different varieties, which may be 

 classed as early maturing, late maturing, nonmaturing, and sterile or 

 nonflowering, are due primarily to responses to different day lengths 

 which come into play as the season advances. Here, again, low temper- 

 ature becomes a factor of increasing importance as the season advances, 

 and, so far as concerns "short-day" plants, it controls the situation with 

 respect to the conditions of nonripening of fruit and of nonflowering. 

 With increasing latitude this relationship between the opposed action of 

 the length of day and the falling temperature becomes more critical for 

 the later maturing varieties. With decreasing latitude a condition is 

 reached in the subtropics which is much more favorable to late maturing 

 or short-day varieties, for the length of day may fall below the critical 

 maximum for flowering without the inhibiting or destructive action of 

 low temperatures coming into play to prevent successful fruiting. At 

 the equator, annual periodicity of both temperature and length of day 

 cease to play an important r61e in plant processes. 



LENGTH OF DAY AS A FACTOR IN THE NATURAL DISTRIBUTION 



OF PLANTS 



In an intelligent understanding of the natural distribution of plants 

 over a particular area those factors which are favorable or unfavorable 

 to growth and successful reproduction for each species must be given 



