THE SOURCES OF CARBON DIOXIDE 329 



while in both cases the volume of gas in the receiver increases proportionately 

 to the amount of organic acid decomposed 1 . 



Although no absolute proof has been given, it can hardly be doubted 

 that this special production of oxygen takes place at the expense of 

 the accumulated organic acids. Decisive evidence as to whether the acid 

 in the cell-sap may be oxidized to carbon dioxide without the aid of 

 the chloroplastids, might be obtained by inducing in them a condition 

 of assimilatory inhibition, or by exposing an etherized plant to light. The 

 more active decomposing action exerted by the less refrangible rays 

 affords no conclusive evidence one way or the other 2 , but even if the 

 chloroplastids do actually produce carbohydrates directly from organic 

 acids, still this does not support LiebigV 5 contention that the formation 

 of organic acids is the first step in the assimilation of carbon dioxide. 



The accumulation of acid may be very pronounced, and thus Kraus 

 (I.e. 1884, p. 19) found that i cubic centimetre of the sap of BryopJq 'Hum, 

 expressed from a plant kept in darkness, neutralized 5-5 cubic centimetres 

 of a c-ooi per cent, solution of sodium hydrate, whereas after being 

 exposed to light i cc. of the sap neutralized only 0-45 cc. of the soda 

 solution. According to Mayer (1. c., 1878, p. 287) 28 grammes of acid leaves 

 of BryopJiylluui when exposed to light may produce 40 cc. of oxygen, 

 derived solely from the organic acid which they contain. General summaries 

 are given by Warburg (1. c., p. 144), and by Aubert (Rev. gen., 1. c., p. 422). 



SECTION 57. The Sources of Carbon Dioxide and the Optimal 



Percentage for Assimilation. 



The structural arrangements have already been described by means 

 of which the plant is able to obtain a sufficient supply of carbon dioxide 

 in spite of the small amount present in the surrounding air, and it is 

 owing to these special adaptations that a leaf or green stem absorbs directly 

 from the atmosphere 4 almost the whole of the carbon dioxide which it 

 assimilates. The amount obtained from the roots and deeper parts of the 

 stem is insufficient to cause any perceptible production of starch, as Moll 

 found when the stem and leaves of a plant were kept in an atmosphere free 

 from carbon dioxide, but its roots were in a humus soil. Minute quantities 

 of dissolved gas may, however, reach the leaves from the roots, and de 

 Saussure 5 showed that traces of oxygen may be produced when no other 



1 For examples of the changes occurring in the composition of the enclosed air under varying 

 conditions, see Aubert, 1. c., pp. 443, 559. 



- G. Kraus, I.e., 1884, p. 20; 1886, p. 43, according to whom the opposite results given by 

 other authors are not applicable to the point at issue. 



3 Liebig, Die organ. Chem. in Anwend. auf Agric. u. Physiol., 1840, p. 26, and 1876, pp. 17, 31. 



4 Recognized by Saussure, Rech. chim., 1804, p. 51. 



5 Saussure, I.e., pp. 112, 122; Boussingault, Agron., Chim. agric., &c., 1868, T. IV, p. 294. 



