Finally between the lamp and the cylindrical vessel is 

 a bowl D. containing a solution of alum to arrest the 

 heat-radiation of the lamp. 



§ 2. Préparation of the water and déter- 

 mination of the dissolved COo. 



For some preliminary expérimenta tapwater was used, 

 which after having been thoroughly boiled (to expell the 

 dissolved oxygen) was cooled in an atmosphère of COo 

 and then poured into the réservoir B. In fact an active 

 absorption of COo took place in water prepared in 

 this way. 



Yet, in such water the détermination of the free and 

 the half-free COo, both a source of the COo-assimilation 

 (as contrasted with the COo from the carbonate) is rather 

 a protracted process because of the simultaneous présence 

 of magnésium-carbonate and calcium-carbonate. In order 

 to save time some experiments were made with distilled 

 water similarly prepared. However it was obvious, that 

 by using this water the Helodeashoots did not assimilate 

 nearly as well as in the prepared tapwater, notwithstan- 

 ding the distilled water also contained a great quantity 

 of dissolved COo. Most probably this decrease is to be 

 attributed to the absence of half free COo in the distilled 

 water; for, when a quantity of potash, almost équivalent 

 to the carbonate in the tapwater, was added before 

 bubbling the COo through the distilled water, the CO^- 

 assimilation took place with the same intensity as in 

 the tapwater. 



Àfterwards I found that Àngelstein ') had also 

 observed that in distilled water the assimilation-process of 

 waterplants is nearly stopped and that it begins when 

 KHCO3 is supplied. 



h Cohns Beitr. z. Biologie d. Pfl. 9. 1911, S. 93. 



