PASSAGE OF GASES THROUGH CELLS AND CELL-WALLS 183 



SECTION 30. The Passage of Gases through Cells and Cell-walls. 



Except in the case of primordial cells, gases must pass through cell- 

 walls to reach the living protoplast. The permeability of the cell-wall is 

 dependent upon its thickness and specific quality, and also upon whether it 

 is dry or saturated with water. In living turgid cells the walls are entirely 

 or partially saturated, and hence the consideration of the gaseous exchanges 

 through membranes containing water of imbibition is of the utmost im- 

 portance. Amongst such the cork and cuticle are included, although these 

 retain less and less water as the impregnation with waxy substances becomes 

 more and more complete (Sect. 21). 



As a general rule the loss of water renders gaseous exchange increas- 

 ingly difficult ; Wiesner found that completely dried cellulose walls were 

 quite impermeable to gases, while in cuticularized, suberized, and lignified 

 cell-walls the permeability merely underwent a pronounced decrease on 

 drying 1 . Probably absolute impermeability is never reached, for the nega- 

 tive pressure originated in parenchymatous cells by drying disappears 

 after a time. Wiesner's results may therefore be regarded as coinciding 

 with those of Lietzmann' 2 , who found that only a marked diminution of 

 the permeability took place in cellulose and other membranes under the 

 conditions mentioned. 



No attention need be paid to the contradictory older researches, since 

 the different results obtained might have been due to a formation of 

 minute slits or to other causes. It is, however, quite possible that in 

 different cell-walls specific differences exist, or that the permeability to 

 gases may be increased by a formation of pores or fissures during drying, 

 for under certain conditions a gelatine film cracks as it dries, though 

 not when dried after fixation in alcohol. Further critical researches are 

 required to decide whether, as different workers state, the walls of the 

 tracheae do actually become more permeable to gases as they dry 3 . 



The cell-wall therefore behaves similarly to a film of gelatine, whose 

 permeability to gases rapidly decreases as it dries. A plate of non- 

 swelling gypsum behaves very differently, for as it dries its pores fill with 

 air, and hence gases diffuse in mass and with much greater rapidity than 

 was the case at first. It is apparently owing to an impregnation with 



1 \Yiesner u. Molisch, Sitzungsb. d. \Vien. Akad., 1889, Bd. xcvill, Abth. i, p. 712. [Kamer- 

 ling (Zur Biol. u. Physiol. d. Zellmembran, Bot. Centralbl., 1897, Bd. LXXII, p. 49) finds that in the 

 interior of living dried moss-cells a vacuum is maintained for an almost indefinite length of time.] 



2 Lietzmann, Flora, 1887, p. 339. 



3 Of the literature given by Lietzmann and Wiesner, may be mentioned here : Barthe'lemy, Ann. 

 d. sci. nat.. 1868, v. ser., T. ix, p. 287, and 1874, v. sen, T. xix, p. 138 ; X. J. C. Mu'ller, Jahrb. f. wiss. 

 Bot, 1869-70, Bd. vn. p. 175. On tracheal walls, see \Viesner, Sitzungsb. d. Wien. Akad., 1879. 

 Bd. LXXIX, Abth. i, p. 33 (sep.) ; Drude, Studien iiber d. Cons.-methode d. Holzes, 1889 ; Stras- 

 burger, Uber Bau u. Vorrichtungen d. Leitungsbahnen, 1891, p. 711. 



