EXTERNAL COMMUNICATIONS OF THE A ERI FERGUS SYSTEM 189 



stomata in the epidermis, and of intercellular passages through lenticels or 

 through spongy cork in the periderm. These are not found in all plants, 

 but become necessary whenever gaseous exchange is rendered difficult by 

 the presence of comparatively impermeable external walls developed as 

 a protection against excessive transpiration (Sects. 27, 29). 



According to Klebahn 1 , intercellular air-channels are commonly present 

 in the periderm, though they are often feebly developed. It is by a localized 

 production of spongy cork that the lenticels found on branches, stems, 

 and roots are formed, and wherever stomata were originally present, the 

 lenticels commonly appear beneath them, so that the aeriferous system 

 remains provided with a point of exit at the same spot. 



Stomata are found almost without exception upon sub-aerial organs 

 only, and in greatest abundance upon the green leaves 2 . This is ap- 

 parently because they are partly for the purpose of enabling the traces 

 of carbon dioxide present in the air to be fully utilized, for, as has already 

 been shown, when the stomata are closed the absorption of this gas is 

 commonly insufficient to permit any formation of starch, whereas oxygen 

 in most cases still penetrates in sufficient quantity for all requirements. 

 Hence it is that stomata are scarce or absent in organs or plants with but 

 little or no chlorophyll 3 . The presence of stomata is, however, apparently 

 advantageous when marked surface development (as in petals) induces the 

 formation of a relatively impermeable cuticle as a protection against 

 transpiration. 



To give a detailed account of the presence and distribution of the 

 stomata is beyond our purpose 4 . It is well known that in leaves frequently 

 one surface, usually the under one, is more richly provided with stomata 

 than the other, and this surface may indeed be the only one which bears 

 them. In all cases the well-developed intercellular space-system ensures that 

 carbon dioxide shall be conveyed in sufficient amount to the more highly 

 chlorophyllous cells of the upper surface, and the importance of these 

 tiny but numerous openings in gaseous exchange is shown by the fact 



1 Klebahn, Die Rindenporen, 1884 (Sep.-abdr. a. d. Jenaischen Zeitschr. f. Naturwiss., Bd. x). 

 On lenti-cells, see cle Bary, Comp. Anatom., 1877, p. 575 ; Haberlandt, Physiol. Pflanzenanat, 1896, 

 2. Aufl., p. 407. On the spongy cork of aeriferous breathing-roots, see the literature given in 

 Sect. 29; also A. Weisse, Ber. d. Bot. Ges., 1897, p. 303. [A. Wider, Die Function d, Pneu- 

 mathoden u. d. Aerenchymes, Jahrb. f. wiss. Bot., 1898, Bd. XXXII, p. 503.] 



a In Musci, stomata are present only on the spore capsule. On the development of the peculiar 

 stomata of the Marchantiaceae, see Leitgeb, Sitzungsb. d. Wien. Akad , 1880, Bd. LXXXI, Abth. i, 

 p. 40. 



3 Johow, Jahrb. f. wiss. Bot., 1889, Bd. xx, p. 506. On the occurrence of stomata on subter- 

 ranean organs, see Hohnfeldt, Bot. Jahresb., 1880, Bd. I, p. 48; Kohl, Transpiration d. Pflanzen, 

 1886, p. 26. 



4 A general account is given by de Bary, Comp. Anat, 1877 ; Haberlandt, I.e.; Tschirch, 

 Pflanzen-anat., 1889, p. 431. Also A. Weiss, Sitzungsb. d. Wien. Akad., 1890, Bd. XCIX, Abth. i, 

 p. 307 ; Wagner, ibid., 1892, Bd. cr, Abth. i, p. 513. 



