332 THE FOOD OF PLANTS 



of this gas is sufficient to produce the maximal activity of carbon dioxide 

 assimilation. 



Kreusler ', by using the constant illumination of the electric light and maintain- 

 ing a constant percentage of carbonic acid gas, estimated for the leaves of Rubus, 

 Carpinus, Tropaeo/nm, &c., the approximate assimilatory activity in atmospheres 

 containing varying amounts of carbon dioxide. The middle column of the following 

 table indicates how much more of this gas was present than in ordinary air. 



Amount of C0 2 . Relative assimilatory activity. 



Percentage. In air = 1. In air = 100. 



0-03 i 100 



0-06 2 127 



o.n 3-5 185 



0-56 17 209 



7.26 220 230 



14.52 440 266 (?) 



The optimal percentage appears to lie at about 10 per cent. Godlewski 2 

 obtained similar results, while, as might be expected, under strong illumination the 

 optimal percentage is somewhat higher. It is probable that during the period 

 of coal-formation the air contained more carbon dioxide than it does now, 

 and the luxuriant vegetation of this epoch was perhaps partly due to this fact 

 (Sect. 51). The optimal percentage for growth is not the same in all cases; 

 Montemartini found that plants grew best in air containing 4 per cent, of carbon 

 dioxide, while de Saussure found that the growth of peas was retarded in the 

 presence of 8 per cent, of this gas ; results obtained by other observers show that 

 green plants are ultimately injured by an atmosphere containing 4 to 15 per cent, 

 of this gas, in which at first assimilation is most active R . Under normal conditions 

 the carbon dioxide hardly ever accumulates to an injurious or fatal extent in the 

 air, whereas in soil and putrefying water this may often occur 4 . It is not the 

 percentage amount of carbon dioxide but its partial pressure that is all important, 

 and Boussingault and Bohm have shown that when mixed with an indifferent gas 

 under pressure a small percentage may act injuriously : \ 



1 Kreusler, Landw. Jahrh., 1885, Bd. xiv, p. 951. 



Godlewski, Arb. d. Hot. Inst. in Wiirzburg, 1873, Bd. I, p. 343. Schutzenberger (Compt. 

 rend., 1873, T. i.xxvn, p. 272) obtained similar results with water-plants. 



3 Montemartini, quoted by Lopriore, Jahrb. f. wiss. Bot., 1895, Bd. XXVIII, p. 539; Saussure, 

 Rech. chim., 1804, p. 29 ; Davy, Elements of Agric. Chem., 1821, 3rd ed., p. 205; de Vries, Landw. 

 Jahrb., 1879, Bd. vn I, p. 41 7. See also the literature given by Lopriore, I.e., p. 571 ; Mangin, Compt. 

 rend., 1896, T. cxxn, p. 747, and Etude s. 1. vegetation d. s. rapports avec 1'aeration du sol, 1896. 



4 On the carbonic acid of the soil cf. Sect. 28. Influence on the growth of roots : Jentys, 

 Bull. d. 1'Acad. d. sci. d. Cracovie, July, 1892. 



"' Boussingault, Agron., &c., 1868, T. iv, p. 286; Bohm, tlber Bildung von Sauerstoff durch 

 grime Landpflanzen, 1872, p. 18 (Sep.-abdr. a. Sitzungsb. d. Wien. Akad., Bd. LXVI, Abth. i). Cf. 

 also N. J. C. Miiller, Bot. Unters., 1876, Bd. i, p. 353. 



