LABORATORY FOR PLANT PHYSIOLOGY. 51 



Total swelling of living sections of Opuntia in salt solutions at 0.01 M. 



Auxographic measurements, in percentages of original thickness: 



NaCl, 140 KCl, 145; CaCl2, 160; water, 160. , 



Relative amounts of water absorbed from 0.01 M salt solutions by agar-pectin-lecithin cells, con- 

 tents 20 p. ct. sugar solution, measured by endosmose and displacement: 

 Sea water <NaCl< water <KCl<CaCl2. at O.OIM. 

 Relative amounts of water absorbed from O.OIM salt solutions by agar-pectin-gelatine cells, 

 contents 20 p. ct. sugar solution: 



NaCl<water<KCl<CaCl2. at O.OIM. 

 Relative amounts of water absorbed from O.OIM salt solutions by agar-pectin-lecithin-gelatine 

 cells, contents 20 p. ct. sugar solution: 



KCK sea-water <NaCl< water <CaCl2. at O.OIM. 

 Relative absorption of ions from immersion liquids at O.OIM by agar-pectin-gelatine-lecithin 

 cells, contents 20 p. ct. sugar solution: 

 Conductivity tests. 



NaCK KCK sea-water <CaCl2. at O.OIM. 

 Relative penetration of ions into contents of agar-pectin-lecithin-gelatine cells, contents 20 

 p. ct. sugar solution: 

 Conductivity tests. 



KCK CaCl2 < NaCK sea-water. 

 Ionic mobilities. 



K64.6....Ca51. Na 43.6. 



PHOTOSYNTHESIS AND METABOLISM. 



An Improved Eledrometric Method of determining Carbon Dioxide for Photo- 

 synthesis Investigations, by H. A. Spoehr and J. M. McGee. 



On account of the fact that in determining photosynthetic rates it is 

 essential that consideration be also given to the rates of respiration, it has 

 been found that a method based upon the differential determination of the 

 concentration of CO2 in the medium surrounding the plant affords the easiest, 

 as well as the most accurate, method of determining the rate of carbon- 

 dioxide fixation by the plant. 



The general principle of the electrometric determination of CO2, as used 

 in investigations on respiration and photosynthesis, has been described in 

 previous reports; it has since been amplified and improved to yield more 

 accurate results, so that 0.002 per cent of CO2 by volume can be determined 

 with certainty. In employing this method, experience has shown that a 

 number of factors must be given careful consideration. The practice of using 

 single excised leaves, while yielding satisfactory experimental results, demands 

 the determination of exceedingly small quantities of CO2. It is, moreover, 

 often necessary to increase the CO2 concentration of the air-stream 5 to 20 

 times that of normal air, and to reduce the periods of determination of photo- 

 synthetic rates to as short a time as possible. The problem then resolves 

 itself into absorbing so completely the relatively large amounts of CO2 in 

 the air-stream that the very slight differences in the CO2 exchange of the leaf 

 can be determined with sufficient accuracy. 



It is, therefore, essential that the volume of the absorbing solution and 

 the measurements of resistance are commensurate with the accuracy required. 

 Several conductivity cells have been devised for various degrees of accuracy 

 desired. In order to construct a resistance-concentration curve of sufficient 

 accuracy, it was necessary to determine the concentrations of the different 

 solutions of Ba(0H)2 more accurately than could be done by titration with 

 HCl. This was done by allowing definite quantities of the Ba(0H)2 solu- 

 tion to absorb a known quantity of CO2. On the basis of Benedict's extensive 

 analyses, dry air from open-air sources differs by less than 0.002 per cent. 

 The atmosphere was thus used as a very dilute solution of CO2 of known con- 



