206 THE MECHANISM OF GASEOUS EXCHANGE 



to light, the oxygen perceptibly increases in amount. A similar increase is 

 perceptible in the seed-pods of Pisitm and Colutea when exposed to light \ In the 

 bladders of Fucaceae, &c., the amount of oxygen present may increase much more 

 markedly, even to as much as 36 per cent. 2 , while in spite of the presence of open 

 stomata an abundance of oxygen may accumulate in the leaves of plants belonging 

 to the Crassulaceae (Aubert, 1892, I.e., p. 275). 



When respiration alone is possible, the amount of oxygen present decreases 

 to a greater or less extent, while the percentage of carbon dioxide increases. Thus 

 Devaux found (I.e., 1891, p. 352) in the intercellular air of the roots of Daucas 

 carota 77-3 to 89 per cent, nitrogen ; 0-4 to 10-6 per cent, oxygen ; 1-4 to 17-8 per 

 cent, carbon dioxide. Similar numbers have been found for bulbs, tubers, &c. 

 Hence it is easy to see that the amount of oxygen present may not suffice to 

 satisfy all requirements when the respiratory activity is unusually pronounced. 

 Even in the roots of Nnphar hiteum, in spite of their well-developed intercellular 

 system, the oxygen may, according to Uutrochet 3 , sink as low as 8 per cent. 



The air enclosed in the vessels and other tissue-elements of the wood, according 

 to Kruticki, does not differ in composition very greatly from that of the atmosphere, 

 except that in winter it is richer in carbon dioxide and poorer in oxygen. Faivre and 

 Dupre", however, obtained precisely opposite results as regards the air in the vessels 

 of Morns and Vitis 4 . These apparent contradictions can, however, be reconciled 

 with one another, for in all probability when the air in the vessels becomes rarefied 

 a gas will at first collect which is relatively rich in both oxygen and carbon dioxide. 



1 Observed first by Ingenhousz, Versnche mit Pflanzen, 1788, Bd. n, p. 58. Of the works 

 carried out on these and other plants, the following may be mentioned : Saussure, Ann. d. chim. et 

 phys., 1821, T. xix, p. 150; Calvert et Ferrand, Ann. d. sci. nat, 1844, iii. ser., T. in, p. 377; 

 Gardner, Froriep's Neue Notizen, 1846, Bd. xxxix, p. 323; Erdmann, Jahresb. d. Chem., 1855, p. 727 ; 

 Baudrimont, Compt. rend., 1855, T. XLI, p. 178 ; Martin, ibid., 1866, T. LXII, p. 737 ; Saintpierre et 

 Magnien, ibid., 1876, T. LXXXIII, p. 490 ; Joulin, Bot. Centralbl., 1881, Bd. V, p. 102 ; Peyrou, ibid., 

 1891, Bd. XLV, p. 217 ; Devaux, Ann. d. sci. nat., 1891, vii. ser., T. xiv, p. 297 ; Aubert, Rev. gen. 

 d. Bot., 1892, T. iv, p. 275. 



2 Aime, Ann. d. sci. nat., 1841, iii. ser., T. n, p. 536 ; Wille, Bot. Jahresb., 1889, p. 226. 



3 Dutrochet, Memoires, &c., Bruxelles, 1837, p. 175. 



* Kruticki, Bot. Centralbl., 1889, Bd. xxxix, p. 30; Faivre et Dupre, Ann. d. sci. nat., 1866, 

 v. ser., T. vi, p. 366. Bischoff's statement that the air of the vessels is rich in oxygen (De vera 

 vasorum plantarum structura et functione commentatio, 1829, p. Si) is without value, owing to the 

 faulty experimental methods employed. 



