STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 33 



125 c. c. of solution (the amount used in the absorption tubes) can 

 easily be calculated and used as the abscissa of the curve just men- 

 tioned. Thus, after the observed resistance value of a barium- 

 hydroxide solution had been converted into the correct value of 

 specific resistance by correcting for the cell-constant, the CO2 

 equivalent could be directly read from the chart. The difference 

 between this, and the CO2 equivalent of the original solution gives 

 the amount of carbon dioxid emitted during the period the air passed 

 through the barium-hydroxide solution. This chart is reproduced 

 in figure 6. By this means the respiration rates were determined 

 with very satisfactory accuracy and consumption of a minimum 

 amount of time. 



4. The Analyses. 



For each experiment 15 to 20 mature leaves were cut. One-half 

 of these were taken for immediate analysis; the rest of the leaves 

 were analyzed after the respiration experiment. Both sets were of 

 course treated exactly alike. Dry-weight determinations were made 

 in duplicate. The leaves were folded up and placed in wide-mouth 

 weighing bottles, the covers placed on the bottles, and weighed. 

 The weighing bottles, with the covers on, were then placed in an 

 electric oven at 98° to insure rapid kiUing. After about 30 minutes 

 the covers were removed and the material was dried at the same 

 temperature for 24 hours. The weighing bottles were then placed 

 in a desiccator and weighed. This dry material was then ground in 

 an agate mortar to a fine powder, after which it was ready for analysis. 



For the determination of the total sugars, 0.5 gram of the leaf 

 material was hydrolyzed for 3 hours in 25 c. c. of 1 per cent hydro- 

 chloric acid, filtered, neutralized with sodium bicarbonate made up 

 to 100 c. c, and the sugars determined with an alkaline copper- 

 sulphate solution by a method the details of which have already 

 been described. ^ Although this method entails more work than some 

 of the other methods in common use, it was found that these pre- 

 cautions were justifiable in order to obtain accurate results with 

 plant material. 



The determination of total sugars seemed to yield sufficient infor- 

 mation relative to the carbohydrate-supplj^ of the leaf, especially 

 in the light of previous investigations which showed that with ample 

 water-supply the carbohydrate balance was in favor of the simpler 

 sugars, hexoses and disaccharides. Special experiments showed 

 that the method of hydrolysis used (1 per cent hydrochloric acid) 

 affected only the reserve starch and not the structural elements, the 

 cellulose of the leaf. It could therefore be assumed that the values 

 for the total sugars give an indication of the amount of carbohydrate 

 material which was available to the leaf for respiratory purposes. 



1 Spoehr, H. A. Carnegie Inst. Wash. Pub. No. 287, 31 (1919). 



