criticised in that publication, partly because the results, 

 to be mentioned further on, contain a criticism on many 

 of thèse observations. 



When the influence of température on a process is to 

 be estimated accurately, the velocity of that process at a 

 certain moment should be determined as accurately as 

 possible. But this is only to be obtained when the velocity 

 is constant. When this is not the case, one has to be 

 contented by measuring the mean velocity during a space 

 of time which should be taken as short possible. 



Now we will describe a method which will enable us 

 to détermine the mean velocity of assimilation during a 

 very short space of time (± 5 minutes). This velocity was 

 determined by measuring titrimetrically the amount of 

 oxygen formed by the assimilation. Meanwhile care had 

 to be taken that the amount of available carbondioxide 

 should be constant and that this gas should be présent 

 in so great a quantity that it did not diminish too much 

 during the assimilation, in other words that the amount 

 of carbondioxide could not be a limiting factor in the 

 meaning attached to it by Blackman^). Likewise the same 

 care had to be taken in référence to the available energy 

 of light. In thèse circumstances it might be expected that 

 a change of the COo-assimilation during variation of tem- 

 pérature, was exclusively caused by this factor. 



Shoots of Helodea canadensis, which had been put up 

 in the laboratory for some time before the beginning of 

 the experiment, were used as testing material. Water con- 

 taining carbondioxyde, but no oxygen was streaming along 

 thèse shoots, and the oxygen formed in the light of an 

 electric lamp was measured. 



In a treatise, entitled: „Ueber den Gasaustausch der 

 Wasserpflanzen. Ein Beitrag zur Kritik der Blasenzâhl- 



') Ann. of Botany. 19, 1905. p. 281. 



