1 88 GROWTH 



and the effect of any one such factor on growth is the result- 

 ant of its action on the opposing components of growth. 

 Thus temperature accelerates respiration and also car- 

 bon 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 result. In illustration, 

 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. In conditions of 

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

 it was found that measurable growth occurred at o° C. and 

 that the greatest rate was at 28 C. Van't Hoff's law is 

 applicable only for small ranges of temperature and, in 

 general terms, the temperature coefficient decreases with a 

 rising temperature. 



The same general observations have been recorded for 

 lower plants. Thus Fawcett % found that the radial growth 

 rate of cultures of the fungus Pythiacystes atrophthora remained 

 constant as long as the environmental conditions did not 

 change. With increasing temperature a greater rate of growth 

 is exhibited up to a certain degree ; when this is passed, the 

 rate of growth falls off. Thus — 



At io° C, radial growth rate = 2-5 mm. per 24 hours. 

 ., 20 C, „ ,, ,, = 6-o ,, ,, ,, 



,, 2o V/. f u t) n "75 " " >> " 



,, 33 C, ,, ,, >> = 2 o ,, ,, ,, ,, 



In Aspergillus niger Terroine and Wurmser § found that 

 the influence of temperature on the rate of growth varies with 



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



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



% Fawcett : " Johns Hopkins Univ. Circ," 1917. *93- 



§ Terroine and Wurmser : " Bull. Soc. Chim. Biol.," 1922, 4, 519. 



