EFFECT OF TEMPERATURE 121 



consequences of the known environmental conditions, provided 

 that proper regard is paid to the distinction in time between 

 the incidence of the conditioning factor and its manifestation in 

 the crop. Thus the daily fluctuations in the flowering curve 

 are predetermined and controlled by the weather conditions 

 which obtained a month before the flower opens. This is the 

 principle of predetermination : its importance is obvious ; an 

 accurate knowledge of predetermining factors will amongst 

 other things give to certain aspects of physiology, ecology and 

 agriculture a precision which now is sometimes lacking. 



THE CONDITIONING FACTORS. 



TEMPERATURE. The factors which condition growth and 

 its rate are precisely those which influence anabolic and 

 catabolic activities, and the effect of any one such factor on 

 growth is the resultant of its action on the opposing com- 

 ponents of growth. Thus temperature accelerates respiration 

 and also carbon assimilation, but since in a vigorous green 

 plant the products of one hour's carbon assimilation is a 

 sufficient provision for many hours' respiration,* there will be 

 a balance on the credit side ; wherefore an increased temperature 

 will result in an increased rate of growth, other things being 

 equal. But there is a limit to the height of the temperature; 

 if a certain degree be exceeded, growth will be retarded, come 

 to a halt, and ultimately a decrement will obtain. In illus- 

 tration, it is a common laboratory experience that the increase 

 in length of the radicle of a seedling is a linear function of time 

 and that there is a gradual increase in the rate of growth up to 

 about 28 C. Talma f observed the growth of the radicle of 

 Lepidium sativum from o to 40 C. Under conditions of 

 constancy of temperature for at least three and a half hours, 

 it was found that measurable growth obtained at o C. and that 

 the greatest rate occurred at 28 C. Van't Hoff's law is appli- 

 cable only for small ranges of temperature and, in general 

 terms, the temperature coefficient decreases with a rising 

 temperature. 



*See Boysen-Jensen : " Bot. Tidsskr.," 1918, 36, 219. 

 t Talma: " Koninkl. Akad. Wetensch.," 1916, 24, 1840. 



