76 



does not show a marked optimum; but it is like the curve 

 found by Czapek. The lowest value is 2'20'' at 25° C, 

 but at 30° C. we find 3'30" and at 35° C. 2'30". The opti- 

 mum hère lies at 25° C. When tracing the thin line, 

 running through the points representing the presentation- 

 time after 12 hours' preliminary warming, we find a 

 marked optimum at 30° C. ; the values just mentioned 

 are in that case: 2'20", l'40", 5'. This slight displacement 

 was the only variability of tho optimum that could be 

 obtained. 



Summarising we see that the time-factor makes itself 

 felt very markedly, not only with regard to the harmful 

 influence, but also as regards the favourable influence 

 of the high températures. The variability of the optimum 

 with the time of observation is only very small. 



§ 14. Limiting factors. 



I briefly repeat hère what was said in 4? 1 about 

 Blackman's views on limiting factors. When tracing the 

 influence of some factor upon a physiological process, 

 which process increases as to its intensity when that 

 factor is increasing, we arrive at a point, where a further 

 increase of the factor mentioned does not give rise to a 

 further increase of the intensity of the process, because 

 some other factor in the process puts a limit to further 

 increase. In S 1 the assimilation in its relation to the 

 amount of CO2 available was quoted as an instance (cf. 

 flg. 2 on p. 6). When the amount of CO2 is increasing, 

 assimilation will also increase, until a point is reached, 

 where the amount of radiant energy available (when we 

 suppose this factor to remain constant during the experi- 

 ments) will allow no greater assimilation. From that point 



