THE PRODUCTS OF THE ASSIMILATION OF CARBON DIOXIDE 321 



primary assimilatory product, for Hillburg showed that the quantity of ' hypochlorin ' 

 present does not decrease in an atmosphere free from carbon dioxide \ 



The almost instantaneous stoppage and recommencement of the move- 

 ment of the bacteria employed as a test for oxygen shows that its evolution 

 commences immediately the green parts are exposed to light, and ceases as 

 soon as the illumination fails. The entire process probably takes place 

 extremely rapidly, for G. Kraus 2 found that a perceptible amount of starch 

 appeared in a starchless Spirogyra filament after five minutes' exposure to 

 light. In other plants starch is not formed until some hours have elapsed, 

 but this is almost certainly because it is commonly deposited only when 

 the primary products of assimilation have accumulated to a certain extent. 



It is not known at what stage in the assimilation of carbon dioxide 

 the oxygen is set free, although various facts show that oxygen and not 

 ozone is liberated. No ozone or hydrogen peroxide is present in the interior 

 of the cell, and Pringsheim's supposition that the oxygen is excreted in the 

 form of a compound which decomposes outside or on the surface of the 

 cell, is certainly erroneous. The liberated oxygen diosmoses rapidly to all 

 parts where a lower partial pressure exists, and hence escapes from the cell 

 before any marked accumulation or condensation is possible 3 . 



In the assimilation of carbon dioxide, allowing for the gaseous ex- 

 changes due to respiration, slightly more oxygen appears to be produced than 

 would be the case if the following simple equation represented the actual 

 process: 6CO 2 + 6H 2 O = 6O 2 + C G H 12 O 6 (glucose). This does not, however, 

 indicate that the primary products of assimilation are not carbohydrates, for 

 a similar excess of oxygen would appear if a little organic acid continually 

 appeared in metabolism and was decomposed in light with an evolution of 

 oxygen (Sect. 56). Even the first visible product of assimilation does not 

 remain intact, but may continually be converted into other substances ; thus 

 Saposchnikoff 4 found that only 64-87 per cent, of the assimilated carbon 

 dioxide appears in the leaves of the sunflower in the form of carbohydrates. 



Boussingault and others have shown that the assimilation of carbonic acid gas 

 usually causes but little change of volume in the enclosed air, but sometimes a very 

 marked difference may be produced. Bonnier and Mangin, and other investigators, 



1 PringsheimJahrb.f.wiss.Bot.,i879-8i,Bd.xn,p.288; Pfeffer,Physiol., i.Ausg.,Bd.i, p. 195. 

 a G. Kraus, Jahrb. f. wiss. Bot, 1869-70, Bd. VII, p. 511; Famintzin (Jahrb. f. wiss. Bot., 

 1867-8, Bd. VI, p. 34) carried out similar researches with Spirogyra at a still earlier date. 



3 [Pringsheim, Sitzungsb. d. Akad. d. Wiss. zu Berlin, 1887, Uber d. Abhangigkeit d. Assim. 

 griiner Zellen u. Sauerstoffathmung. Cf. Pfeffer, Oxydationsvorgange in lebenden Zellen, 1889, 

 pp. 149, 479, and Ewart, Journ. of Linn. Soc., Vol. xxxi, 1891, p. 418. An actively assimilating 

 green cell, if suddenly killed, may evolve faint traces of oxygen for a short time after death, but this 

 is merely oxygen dissolved in the cell-sap, &c. ; see Ewart, I.e., 1897, p. 146.] In Sect. 101 an 

 account is given of the power of occluding oxygen possessed by certain bacteria. 



4 Saposchnikoff, Ber. d. Bot. Ges., 1890, p. 241. On similar researches by Menze, see the 

 .criticism by A. Meyer in Bot. Zeitnng, 1888, p. 465. 



PFEFFKR Y 



