■ 312 Mr. A. J. Wilmott. Experimental Researches on_ 



In the following experiments the initial CO2 effect occurred so regularly 

 and was so obvious to the eye that readings were often omitted during it. In 

 the graphs of many of the experiments figured in this paper it will be seen 

 that the curve of the initial phase has been drawn in without any actual data. 



At the end of the initial phase (some 10 to 20 minutes) the bubble rates 

 become practically constant for a long time, and it is these long constant 

 values which are taken as the measure of the CO2 assimilation possible when the 

 'CO2 in solution or the C02of bicarbonates is limiting. When the investigator 

 changes the conditions every 10 minutes, as Pantanelli sometimes did, it is 

 -clear that the results recorded may be involved in considerable complications. 

 Earlier observers have tried to rush through their experiments as quickly as 

 possible to avoid spontaneous uncontrollable variations in bubble size, but the 

 use of glass bubblers removes all this risk, and experiments can be carried out 

 slowly and constancy assured in each condition before it is changed. 



The uniformity of bubble rate is so regular a happening when a bubbler is 

 used that the observer gets into the habit of taking many fewer readings of 

 bubble rate than would be permissible with a varying cut stem. It will be 

 noticed that in many of the graphs the actual points recorded seem rather 

 scanty, but they are really quite adequate to establish the conclusions drawn 

 when working with this improved technique. 



(C) Apparatus, Experimental Procedure, and Minor So2irces of Error. 



The plants used in subsequent experiments were either Elodea or Callitriche. 

 They were fitted with a glass bubbler and set up, depending from the cork of 

 the .i)ubbling cup in the way shown in fig. 1. The bubbling cup always 

 contained distilled water, and it was fitted into a hole in a thick cork plate, 

 which again fitted into the rectangular glass museum cell used as a plant 

 chamber. For single shoots a chamber of 200 c.c. capacity was used. A hole 

 in the plate near one corner, closed by a cork during experiment, allowed 

 fresh solutions to be poured in through a funnel, while a small piece cut from 

 the opposite corner allowed the liquid to be quickly poured away. In the 

 experiments with acid the required amount of N/IO HCl was added through 

 this hole from a graduated pipette, and stirred by twirling a glass rod which 

 passed to the bottom of the cell and had its lower inch beut at right angles 

 and flattened. The temperature could be read continuously from a thermo- 

 meter passing through another hole in the lid. 



The CO2 solutions used were diluted from a stock standard containing 

 about 12 volumes per cent. CO2, prepared from a Kipp's apparatus and 

 cleansed from acid in the usual way ; the actual strength was determined by 

 titration with excess of baryta and hydrochloric acid. For the avoidance of 



