75 



the CLirve through the values found experimentally after 

 1, 2 and more hour« prcliminary hcating. 



Not only at harmful températures (above the optimum), 

 but even at those températures which are not directly 

 harmful, this phenomenon can give rise to errors. In this 

 case also it is possible, that after one or two hours the 

 reaction-velocity is not yet at its fuU height. In a given 

 case everything dépends upon the relative rapidity of the 

 two opposing influences. If the favourable influence has 

 its full effect almost instantaneously, while the harmful 

 influence on the contrary makes itself felt only gradually, 

 we shall find a state of things similar to the case of 

 assimilation, investigated by Miss Matthaei. If on the 

 other hand both influences work slowly, we get curves, 

 as hâve been found by Kuyper in the case of respiration, 

 and by my self in the case of the geotropic présentation - 

 time. At very high températures the harmful influence 

 is prédominant, and the initial values are considerably 

 lower thau they should be, — were it possible to get the 

 theoretical values after a time zéro by extrapolation. At 

 slightly lower températures we see first the harmful in- 

 fluence and afterwards during a short time a favourable 

 influence, as Kuyper found at some températures, or 

 the favourable influence lasts a long time after a short 

 preliminary harmful influence, as was the case in my 

 experiments at 30° C. When both influences are at work 

 with the same intensity and at the same rate it may be 

 possible to get curves which are nearly horizontal. 



Hitherto I hâve not discussed the question, whether 

 the optimum varies with the time of observation. The 

 curves in Fig. 6, p. 58 answer this question. The presen- 

 tation-time after one hours' preliminary warming has 

 there been indicated bv means of a thick linc. This line 



