38 THE CARBOHYDRATE ECONOMY OF CACTI. 



CO 2 DETERMINATION. 



The method employed for determining the rate of CO 2 emission was 

 based upon the absorption of the carbon dioxid in a standard solution of 

 barium hydroxide and titration of the unchanged base. The spines were 

 first carefully removed from the cactus joints, weighed, and the cut ends 

 were covered with a soft wax preparation to prevent a traumatic effect. 

 The joints (usually 4 to 6 were employed) were placed in a desiccator pro- 

 vided with entrance and exit tubes. The desiccator was then hermetically 

 sealed, and a light-proof cover placed thereover. This was immersed com- 

 pletely in a Freas electric water thermostat and the tubes were connected. 

 The air was drawn through the apparatus by means of a specially devised 

 electric pump and the pressure was regulated by a Palladin pressure 

 regulator. The air passed first through a train of moist soda-lime, then 

 through a 25-foot coil of glass tubing in the thermostat, into the respiration 

 chamber ; from there through a water-trap, and finally through the Meyer's 

 tubes containing 125 c. c. of 0.1 normal barium hydroxide. A series of 

 tubes was so arranged that the stream of air could be passed into a fresh 

 tube without interrupting the experiment. A single tube collected the 

 carbon dioxid for from 4 to 12 hours, and the experiment was allowed to 

 run usually from 48 to 60 hours. The barium hydroxide together with the 

 precipitated barium carbonate was poured into a narrow bottle, sealed, and 

 the carbonate allowed to settle. After 24 hours aliquot portions of the clear 

 solution were drawn off with a pipette and titrated with standard hydro- 

 chloric acid, using methyl orange as indicator. The amount of carbon 

 dioxid per unit time can be easily calculated from the difference in strength 

 of the barium hydroxide solution between the original and after the air 

 from the plant had passed through. 



