232 PHOTOSYNTHESIS 



between the place where the gas is being emitted and the surface of the 

 water must be constant. 



Now the principle of the method lies essentially herein: that the rate 

 of bubble emission is taken as a measure of the rate at which the plant 

 is reducing carbon dioxide and liberating oxygen. In order that bubble 

 formation may take place it is necessary that there be an increase in pres- 

 sure in the intercellular spaces. This pressure must overcome ( 1 ) the 

 frictional resistance of the gas in the minute intercellular canals, (2) the 

 capillary forces of the water in these canals, (3) the weight of the capillary 

 water column, and (4) the pressure of the water from the cut surface 

 to the level of the water. A direct relationship between the rates of 

 bubble emission and photosynthesis will exist only if the composition of 

 the gas is pure oxygen or contains only negligible impurities or if the 

 gas contains always a definite percentage of oxygen independent of the 

 rate of emission. The method has been subjected to considerable experi- 

 mental criticism, particularly by Pfefifer and by Kniep,^^ although little 

 attention has been paid to their findings and many mistakes have been 

 repeated. Pfeffer on the basis of theoretical considerations pointed out 

 that the oxygen content of the emitted bubbles varies in the same direc- 

 tion as the rate at which the bubbles are emitted. This very important 

 question has been thoroughly investigated by Kniep. His results are 

 discussed below. It is very important that the COg-tension in the water 

 should be constant or nearly so during the entire course of the experi- 

 ments which are to be compared. Sachs ran carbon dioxide into the 

 water. An excess of carbon dioxide is to be avoided, however, as the 

 gas will pass through the plant and be emitted, quite independently of 

 photosynthesis. This gas stream may continue for a long time even in 

 the dark.^2 



The best medium is a large supply of tap water to which the plant 

 has free access or better, as Anglestein " has found, a freshly prepared 

 solution of one per cent potassium bicarbonate. 



Kniep,^* by using the very accurate methods of gas analysis of Krogh 

 in which about 3-6 cmm. of emitted gas were analyzed, investigated the 

 relation between the composition of the gas and the rate at which it was 

 emitted. It was found that the oxygen-content of the gas was higher 

 the greater the rate of bubble emission. The bubble rate was regulated 

 by means of altering the light intensity. Thus it developed that with 

 changing light intensity the rate of bubble emission increases and de- 



"Daubney, H., Phil. Trans. London, 126, 149 (1836). Devaux, H., Ann. scien. 

 nat. Bot., Serie 7, 9, 35 (1889). Reinke, J., Bot. Zcitg., 41, 697 (1883). Cloez, S., 

 and Gratiolet, Ann. sci. nat. Bot.. Serie 3, 32, 41 (1851). Pfeffer, W., Arb. d. bot. 

 Inst. Wurzburg, 1, 1 (1871). Kniep, H., Jahrb. unss. Bot., 56, 460 (1915). 



"Van Tieghem, P., Compt. rend., 69, 482 (1869). 



"Anglestein, V., Cohns Beiir. z. Biol, der PH., 10, 87 (1910). 



"Kniep, H., and Minder, F., Zeit. f. Botan.. 1, 619 (1909). Pantanelli, E., 

 Jahrb. zinss. Bot., 39, 167 (1903). 



