190 THE MECHANISM OF GASEOUS EXCHANGE 



that when they are closed the assimilation of carbon dioxide almost entirely 

 ceases. 



The character and position of the points of exit are such as at once to 

 indicate that they have been developed for purposes of aeration. Thus 

 stomata are usually absent in submerged plants, and in parts growing 

 normally beneath the surface of the water l , whereas on those organs 

 which rise above the water, and on the upper surfaces of floating leaves, 

 they may be as abundant as in terrestrial plants. Moreover, by the 

 arrangements for leading away water and other adaptive modifications, any 

 danger that the capillary pores may suck in water and become blocked is 

 usually avoided *. Under ordinary circumstances neither heavy falls of 

 rain or dew, nor even temporary immersion in water, cause any such 

 occlusion of the stomata. 



Just as intercellular spaces may serve special purposes, acting, for 

 example, as secretory reservoirs, so also may the stomata subserve func- 

 tions foreign to their original purpose. Thus water-stomata serve for the 

 excretion of water and watery solutions, while the sparsely scattered stomata 

 which may be present on submerged plants have lost their primitive 

 importance as gaseous channels. Many stomata never become fully 

 developed, or they may barely open, or may soon permanently close 3 . 

 Occasionally resinous masses may block the stomata in the leaves of the 

 Coniferae 4 , and it is perhaps partly owing to the adherence of particles 

 of soot that Coniferae and other evergreen plants with erect leaves, or 

 with leaves bearing stomata on their upper surfaces, grow badly in the 

 neighbourhood of towns and factories. 



The width of the stomatal aperture varies according to the external 

 conditions, and especially important is the fact, which Amici first observed, 

 but which Mohl first definitely established, that as a general rule the stomata 

 pores narrow as turgidity decreases, and become completely closed as soon 

 as any sighs of withering are manifest, or even earlier 5 . In this way the 

 evaporation of water is rendered increasingly difficult, and as far as possible 

 a fatal loss of water is avoided, although at the expense of the function 



1 For literature see Kohl, Transpiration d. Pflanzen, 1886, p. 26; Sauvaugeau, Compt. rend., 

 1890, T. in, p. 313; Goebel, Pflanzenbiol. Schilderungen, 1893, 2. Th., p. 240. 



2 Cf. Tschirch, Anat., 1889, p. 438; Volkens, Ber. d. Bot. Ges., 1890, p. 120 ; Kerner, Pflanzen- 

 leben, 1887, P- 2 66. On the draining away of water, see Sect. 27 and the literature there given. 



3 De Bary, Comp. Anat., 1887, p. 54. Czeck's data (Bot. Zeitung, p. 805) are not applicable 

 to the point at issue, according to Schwendener (Monatsb. d. Berl. Akad., 1881, p. 866). See 

 also Kohl, Transpiration d. Pflanzen, 1886, p. 27. On the closure by cellular outgrowths, see 

 Molisch, Sitzungsb. d. Wien. Akad., 1888, Bd. xcvn, Abth. i, p. 298." According to Stahl, the 

 stomata of various evergreens are closed in winter (Bot. Zeitung, 1894, p. 126). A similar closure 

 commonly accompanies the assumption of the autumnal colouration by the leaves. 



Thomas, Jahrb. f. wiss. Bot., 1865-6, Bd. iv, p. 28 ; Wilhelm, Ber. d. Bot. Ges., 1883, p. 325. 

 5 Mohl, Bot. Zeitung, ^56, p. 697 ; confirmed by Leitgeb, Mitth. d. Bot. Inst. in Graz, 

 1886, p. 125; Schwendener. Monatsb. d. Berl. Akad., 1881, p. 833, &c. 



