MECHANISM AND CAUSES OF PHYSIOLOGICAL COMBUSTION 541 



supposed it to be an obligate anaerobe. Various anaerobes grow in the 

 presence of oxygen when associated with other micro-organisms, but since 

 even then the former must come more or less into direct contact with 

 the gas, it is obvious that they must be influenced in some special manner 

 by the symbiotic association l . 



It is possible that both the minimum and maximum pressures of 

 oxygen for a particular plant may vary to a certain extent according to 

 circumstances, and their precise determination is made still more difficult 

 by the fact that an oxygen-pressure which the plant can withstand even 

 for a prolonged time may ultimately cause death. This is the case not 

 only with aerobes but also with anaerobes, for the latter may continue to 

 move for a long time in the presence of a super-maximal amount of oxygen. 

 Certain plants possess hardly any accommodatory power, and sulphur- 

 bacteria can withstand only slight variations from the optimal percentage 

 of oxygen, whereas many facultative anaerobes are able to live at as great 

 an oxygen-pressure as higher plants. 



The minutest traces of free oxygen are absorbed even by anaerobes. 

 Growth may be inhibited either by a sufficient decrease or increase in the 

 oxygen-pressure, both in Phanerogams and in facultative anaerobes. There 

 can be no doubt, moreover, that the growth even of the most sensitive 

 obligate anaerobes must be possible in the presence of a minimal trace 

 of oxygen, and hence no sharp line of demarcation can be drawn between 

 those anaerobes which continue to grow and those whose growth ceases in 

 the presence of oxygen 2 . 



Each organism is able to withstand a specific density of oxygen, but 

 as is shown by facultative anaerobes the power of anaerobism is not 

 necessarily connected with an excessive sensitiveness to this gas, such as 

 characterizes the obligate anaerobes. The injurious action is peculiar to 

 free oxygen, for this element is present in combined form in all anaerobes, 

 while in the processes of metabolism highly oxidized end-products, such 

 as carbon dioxide, &c., may be produced. The over-concentration of 

 oxygen does not act injuriously by either decreasing or increasing the 

 activity of physiological combustion 3 , for as a matter of fact, growth, 

 respiration, and indeed all vital functions 4 , are at first but little altered. 



1 Kedrowski, Zeitschr. f. Hygiene, 1895, Bd. xx, Heft 3. On Clostridium Pastcitrianuin, cf. 

 Sect. 69. 



2 Beyerinck, Centralbl. f. Bact, 1895, Abth. ii, Bd. i, p. 109, footnote. 



3 Both views have been put forward (cf. Jentys, Unters. a. d. Bot. Inst. z. Tubingen, iSSS, Bd. II, 

 p. 458). The non-oxidation of phosphorus in compressed air affords no criterion as to the action upon 

 a living organism, and in most cases combustion is more active in pure oxygen than in air. Cf. van 

 't Hoff, Chem. Centralbl., 1895, I, p. 676. 



1 As, for example, phosphorescence and heat-production. Cf. de Vrolik et de Vriese, Ann. d. 

 sci. nat., 1839, ii. ser., T. xt, p. 77. 



