530 RESPIRATION AND FERMENTATION 



necessity of diosmosing through the cell-membranes before escape is possible, or 

 by other means l . 



Oxygen being less soluble usually accumulates to a much less marked extent, 

 for the power of entering into loose combination with oxygen is apparently 

 restricted to certain pigment-producing bacteria (Sect. 101). A trace of free 

 nitfogen is normally present in every cell, but apparently merely in dissolved form *. 



SECTION 97. Respiratory Katabolism in Anaerobes. 



Many bacteria are obligate anaerobes, whereas facultative anaerobes 

 can exist in the absence of oxygen only when supplied with suitable food. 

 At a sufficiently high partial-pressure of oxygen all organisms cease to 

 exist, and a descending series of plants exhibiting all grades of specific 

 sensitivity to oxygen may be compiled, which terminates in those obligate 

 anaerobes to which the presence of only a trace of free oxygen is fatal 

 (Sect. 100). Temporary anaerobes in which growth or even division may 

 continue for a time in the absence of oxygen form a transition stage between 

 permanent anaerobes and aerobes, and many organisms which are termed 

 anaerobes, owing to the continuance of growth in the absence of oxygen, 

 may be really only temporarily such 3 . On the other hand, there can 

 be no doubt that permanent anaerobes actually exist, for when a few 

 germs such as those which induce butyric fermentation are sown in 

 a hermetically sealed flask from which all free oxygen has been removed, 

 countless generations succeed one another, and further multiplication ceases 

 only when the food-material is consumed and when injurious waste-products 

 accumulate to an inhibitory extent 4 . 



Saccharomyces cerevisiae is only a temporary anaerobe even under the 

 best conditions, although it may reproduce itself twenty or thirty times 

 in the absence of oxygen 5 . Mucor racemosus possesses a somewhat less 

 marked power of temporary anaerobiosis although it can excite very active 

 fermentation, while M. imtcedo and M. stolo.nifer do not grow in the absence 



1 Cf. Chap. v. Observations on the absorbed gases of turnips by Heintz, Bot. Jahresb., 1873, 

 p. 360. On cucumber seedlings by Laskovsky, Versuchsst., 1874, Bd. XVII, p. 223. Absorption 

 by cell-walls and dead plants, Bb'hm, Ann. d. Chemie, 1877, Bd. CLXXXV, p. 257; Bot. Zeitung, 

 1883, p. 559. 



3 A few details by Deherain et Landrin, Ann. d. sci. nat., 1874, v. se"r., T. xix, p. 165; 

 Compt. rend., 1875, T. LXXXI, p. 198 ; Leclerc, ibid., 1875, T - LXXX, p. 26 ; Bonnier et Mangin, 1. c. 



3 These are Beyerinck's terms (Uber Butylalkoholgahrung, 1893, p. 46). 



4 Beyerinck, I.e., pp. 33, 47 (Sep.-abdr. a. Verb. d. Akad. zu Amsterdam). Beyerinck has 

 recently doubted the existence of permanent anaerobes (Centralbl. f. Bact., Abth. ii, 1897, Bd. ir, 

 p. 41). 



5 This was first observed by Pasteur. Beyerinck, 1893, 1. c., pp. 33, 47. 



