i8o THE MECHANISM OF GASEOUS EXCHANGE 



leaf cannot as a general rule absorb sufficient carbon dioxide when its 

 stomata are closed, and that when its cuticle is highly impermeable it may 

 even be unable to obtain the full supply of oxygen needed for respiration \ 

 The characters of the cork and cuticle differ according to the stage of 

 development, and are subject to modification in correspondence with varying 

 external conditions (Sects. 21 and 38). The diameter of the stomatal pores 

 is moreover changeable, and all passage of gases or water-vapour through 

 them ceases as soon as a falling off in the supply of water causes the 

 stomata to close. Many other factors also influence both the direct and 

 the diosmotic transference of gases, and thus the streaming movements 

 originated by differences of pressure are mainly of importance for gaseous 

 exchange in open air-channels. Moreover, the relative development of the 

 intercellular aeriferous system will naturally largely determine the functional 

 importance of the stomata. 



An account of the very varied kinds of aeriferous systems which may 

 be met with cannot be given here 2 , but a few details concerning the 

 rapidity of gaseous transference through them will be mentioned later. It 

 is obvious that large and well-developed intercellular channels, such as are 

 found in many aquatic plants, will render rapid admixture possible, while 

 when the intercommunicating air-spaces are feebly developed or absent, 

 localized differences in the pressure and composition of the enclosed air 

 may be maintained. According therefore to the degree of development of 

 the intercommunicating channels, as well as to other circumstances, the 

 stomata and lenticels will either be of merely localized importance or will 

 furnish gaseous supplies for distant organs. Thus, in an actively assimi- 

 lating leaf, carbon dioxide is immediately absorbed by the green cells which 

 border upon the stomata (Sect. 57), whereas the well-developed aeriferous 

 system of Nymphaea^ Typ/ia, Equisetnm t and of swamp plants in general, 

 is undoubtedly for the purpose of transferring the necessary oxygen to the 

 rhizomes and roots. A similar functional importance attaches to the erect 

 ' breathing ' roots which frequently arise above the mud of a mangrove 

 swamp 3 , for otherwise roots growing in mud containing anaerobic bac- 

 teria could obtain no oxygen. Even in a stiff clay soil a sufficiency of 



Barthelemy's contradictory assumption (ibid., p. 663) that the stomata are only of subordinate 

 importance in gaseous exchange, do not require any special criticism. 



1 See Mangin, Compt. rend., 1887, T. cv, p. 879. 



2 Cf. Haberlandt, Physiol. Pflanzenanat., 189(1, 2. Aufl., p. 375. 



3 Cf. Jost, Bot. Zeitung, 1887, p. 601 ; Schenck, Flora, 1889, p. 83; Karsten, Mangrove Vege- 

 tation, Bibliotheca Botanica, 1891, p. 55 ; Goebel, Ber. d. Bot. Ges., 1886, p. 249, and Pflanzenbiol. 

 Schilderungen, 1893, n, p. 255. [Wieler (Jahrb. f. wiss. Bot., Bd. XXXII, p. 503) states that the 

 ' pneumathodes ' described by Jost on palm roots are not breathing organs, but are formed, owing to 

 the direct action of water inducing localized hypertrophy. The intercellular spaces are plugged, and 

 are only permeable to water under pressure. That special breathing organs do, however, exist on 

 the roots of many plants cnn hardly be doubted.] 



