i8 2 THE MECHANISM OF GASEOUS EXCHANGE 



reservoirs. The partial or complete filling of dead cells and vessels with 

 air is not usually for the purpose of ensuring more active aeration, for no 

 open communication exists between these aeriferous tissue-elements and the 

 actual intercellular aeriferous system. 



Under normal conditions the aeriferous system is kept free from water, 

 as is indeed necessary for its continued functional activity. The non- 

 wetting of the leaf, and the various contrivances by which rain and dew 

 are led away from it. ensure that the stomata shall not be occluded by water 

 (Sect. 27). When transpiration is active the intercellular spaces always 

 contain air. and it is only when the sap accumulates to excess that they 

 become partially injected with water 1 . In this case, and also when they 

 have been artificially injected, the water disappears from the intercellular 

 spaces - soon after transpiration commences. No noticeable negative 

 pressure is, however, produced in intercellular spaces which are in open 

 communication with the external air, whereas in vessels the loss of water 

 produces a negative pressure tending to suck it in again when the supply 

 is more abundant (Chap. VI). 



Submerged plants are always in a condition of maximal turgidity, 

 and the positive pressure generated by the liberation of oxygen during the 

 assimilation of carbonic dioxide (Sect. 32) tends to keep the aeriferous 

 system open and filled with air. Thus the bladders of Fucus remain tense 

 even when subjected to considerable pressure, and they may burst, owing- to 

 the removal of the external hydrostatic pressure, when the plant is suddenly 

 brought to the surface 3 . Other factors must also influence the formation 

 of intercellular spaces, since these appear sooner or later in the primary 

 meristem, and to a more or less marked extent in organs free from 

 chlorophyll. 



As the external appearance indicates, the volume which the air-spaces occupy 

 varies very much. In the leaves of most terrestrial plants air-spaces form to \ of 

 the entire volume. Unger 4 found a maximal amount of 71-3 per cent, by volume of air 

 in the leaves of the floating Pistia texensis, and a minimal value of 3-5 per cent, in the 

 fleshy leaves of Begonia hydrocotylifolia. In wood the amount of air varies inversely 

 with the amount of water present, as shown by the facts given in Chapter VI 5 . 



1 Cf. on root-pressure, Sect. 41. Additional examples by Westermaier, Sitzungsb. d. Berl. Akad., 

 1884, p. 1107 ; Jonsson, Botaniska Notisera, 1892, p. 252. 



2 Moll, Unters. iiber Tropfenausscheidung u. Injection d. Blatter, 1880, p. 71 (Sep.-abdr. aus 

 Verslagen en Mededeelingen d. Akad. d. Wiss., Amsterdam). Also Barthelemy, Ann. d. sci. nat., ' 

 1874, v - ser., T. xix, p. 167. 



3 Berthold, Mitth. a. d. Zool. Stat. in Neapel, 1882, Bd. Ill, p. 431. On gas vacuoles, see 

 Sect. 22. 



* Unger, Sitzungsb. d. Wien. Akad., 1854, Bd. xn, p. 367. See also Stahl, Uber d. Einfluss 

 d. sonnigen u. schattigen Standorts auf d. Ausbildung d. Laubblatter, 1883, p. 18 ; Aubert, Rev. gen. 

 de Botanique, 1892, T. iv, p. 276. 



5 Estimations by Sachs, Uber d. Porositat d. Tannenholzes, 1877, p. 10 ; Dufour, Arb. aus 

 Wurzburg, 1884, Bd. in, p. 37; Pappenheim, Bot. Centralbl., 1892, Bd. XLIX, p. 36. 



