318 ASSIMILATION. 



838. The amount of carbonic acid unfavorable to assimilation. 



Experiments made by Saussure '■ at the beginning of tliis cen- 

 tury- proved beyond question that plants are not tolerant of an 

 atmosphere containing a large proi:jprtion of carbonic acid. In 

 carbonic acid alone, or even in an atmosphere containing 66 

 per cent of this gas, vegetation was speedily destroyed. It was 

 sliown, however, that if the plants were exposed to full liglit, 

 they could sustain 8 per cent of carbonic acid without injury-. 

 Saussure tliought that the presence of free ox^-gen is necessary 

 to the assimilative work of the leaf. 



839. In 1849, Daubenj'^ carried on an extensive series of re- 

 searches, chief!}' upon plants allied to the dominant vegetation 

 of the Carboniferous period, namelj', ferns and their allies, from 

 which it appeared that even for these plants an amount of car- 

 bonic acid above 10 per cent is injurious. Five species were 

 placed in a receptacle containing about 46 liters of air, and to 

 this air was added one per cent of carbonic acid, and also one 

 per cent daily thereafter, until the amount present reached 20 

 per cent. This proportion was kept for twenty days, small 

 amounts being added, as occasion required, to make up for loss 

 bj' leakage. On the thirteenth day a sensible impairment of 

 the plants was noticed ; and at the end of thirty dajs all of 

 them had been more or less damaged, most having lost their 

 fronds. 



840. Boussingault,' in 1864, conducted a series of experi- 

 ments in order to ascertain whether the presence of free oxygen 

 in an 'atmosphere containing carbonic acid is necessary to the 

 work of assimilation. The results of his researches are given 

 as follows : — 



(1) Leaves exposed to sunlight, in pure carbonic acid, do not 

 decompose this gas, or if at all, very slowlj'. 



(2) Leaves exposed to sunlight in an atmosphere containing 

 a mixture of common air and carbonic acid decompose the 

 latter gas rapidly ; but the oxygen of the air has no part in this 

 operation, since, 



(3) Leaves exposed to sunlight rapidly decompose carbonic 

 acid gas when this gas is mixed with nitrogen, hydrogen, car- 

 bonic oxide, or carburetted hydrogen. 



1 Saussure: Recherohes ohimiques sur la vegetation (Paris, 1804), p. 29. 

 An earlier experiment was made by Percival. 



^ Report of British Association, 1849, p. 56 ; and 1850, p. 159. 

 » Agronomie, iv., 1868, p. 301. 



