196 THE MECHANISM OF GASEOUS EXCHANGE 



stems with a chlorophyllous cortex, the porous cork through which air 

 passes to the periderm layers has not also to provide for the transference 

 of the traces of carbon dioxide to actively assimilating cells, as is the case 

 in the leaf. 



Above all things, it depends upon the development of the aeriferous 

 system whether the gaseous exchanges through stomata and lenticels 

 are mainly of local importance, or are of service to distant parts as well. 

 As a matter of fact, when the aeriferous system is well developed, a slight 

 pressure suffices to drive air through a long piece of stem until it emerges 

 through the stomatal pores of the leaf (Sect. 32). On the other hand, 

 differences of pressure become equalized only slowly in the plants of the 

 Crassulaceae, whereas, according to Devaux, gases circulate more readily 

 in cucumbers, apples, potatoes, and beetroots l . 



Even in plants with well-developed aeriferous systems, gases are not 

 directly conveyed to all the cells, but must travel in solution for a short 

 distance to reach them. It is only by means of diosmosis that an exchange 

 is possible with the tracheides and tracheal vessels which are more or less 

 completely filled with air, for these are not in open communication with 

 the aeriferous system. The intercellular spaces of the medullary rays, 

 however, extend near to or touch the tracheae, and usually communicate 

 with the external air by channels of porous cork 2 . 



That the stomata place the aeriferous system in direct and open communication 

 with the external air may be proved by microscopical observation as well as by 

 direct experiments, such as those conducted by Dutrochet, and later by Raffenau- 

 Delile, Unger, Sachs, &c. The principle of these experiments is essentially the 

 same, though various modifications have been adopted, for in all cases air or some 

 other gas is driven through leaves or leaf twigs, and the exit of bubbles from the 

 leaf or cut surface of the stem or petiole observed under water 3 . The arrange- 

 ments shown in Figs. 22 and 23 are well adapted to demonstrate the transference 

 of gases through the intercellular spaces and stomata. In Fig. 22 the petiole of the 

 leaf, d, passes into the glass cylinder g, which is two-thirds filled with water. If 

 necessary, air-tight connexions are ensured by means of gelatine or cocoa-butter. 

 On placing the tube c in connexion with an air pump, as soon as the pressure in g 

 is lowered to a certain point a stream of air-bubbles comes from the cut surface of 

 the petiole immersed in water. In Fig. 23 the leaf is inclosed in the glass cylinder 



1 Bonnier, Rev. ge"n. de Bot, 1893, T. v, p. m ; Devaux, ibid., 1891, T. in, p. 49, and Ann. 

 d. sci. nat, 1891, vii. ser., T. xiv, p. 309. 



* Amici, Ann. d. sci. nat., 1824, T. n, p. 241 ; Sachs, Ober Porositat d. Holzes, 1877, p. 4; 

 Russow, Bot. Centralbl., 1883, Bd. xill, p. 366 ; Strasburger, Ban u. Verrichtung d. Leitungsbahnen, 

 1891, p. 710, &c. ; de Bary, Comp. Anat., 1877, p. 338; Russow, I.e.; Strasburger, I.e., p. 480; 

 Herbst, Bot. Centralbl., 1894, Bd. LVII, p. 412. See also de Vries, Bot. Zeitung, 1886, p. 788. 



3 Dutrochet, Ann. d. sci. nat., 1832, T. XXV, p. 248, and Mem. p. servir a 1'histoire d. vege'taux, 

 Bruxelles, 1837, p. ^2; Raffenau-Delile, Ann. d. sci. nat., 1841, ii. ser., T. XVI, p. 328; Unger, 

 Sitzungsb. d. Wien. Akad., 1857, Bd. xxv, p. 461 ; Sachs, Kxperimentalphysiol., 1865, p. 252. 



