MECHANISM AND CAUSES OF PHYSIOLOGICAL COMBUSTION 539 



but cherries may contain 1-8 to 2-5 per cent., and seedling peas as much as 5 per 

 cent. Brefeld's observations upon the formation of alcohol by Penidllium were 

 probably not made upon P. glaucum, for according to Diakonow the latter rapidly 

 dies in the absence of oxygen, and Elfving ' did not observe any formation of 

 alcohol by Penidllium glaucum when grown upon readily fermentable media. In 

 addition to alcohol, other substances, including organic acids, are probably produced, 

 and hence no constant relationship necessarily exists between the amount of 

 carbon dioxide and of alcohol -. 



SECTION 100. The Mechanism and Causes of Physiological 



Combustion. 



Respiration is a vital function regulated and maintained by the 

 living organism, which draws neutral oxygen into its metabolism, and by 

 inducing the physiological oxidation of certain substances provides the 

 energy necessary for further metabolic activity and for the continuance 

 of life. The consumption of oxygen is regulated by the requirements 

 of the organism, and when these are satisfied the presence of a surplus 

 produces within certain limits hardly any perceptible effect (cf. Sect. 22). 

 As a matter of fact the respiratory activity of many plants is not markedly 

 modified when the percentage of oxygen is reduced to one-half its normal 

 amount, or when its partial pressure is five to ten times increased. In the 

 latter case an excess of free oxygen collects in the cell, but under normal 

 circumstances it is present both in the protoplasm and cell-sap, as is 

 shown by the existence of living animals within Vaucheria, and by the 

 continued activity of organisms enclosed in the plasmodia of Myxomycetes 3 . 

 Hence all theories fall to the ground which postulate the absence of free 

 oxygen from the interior of the cell, and thus restrict physiological com- 

 bustion to the outermost layer of the cyto-plasm 4 . 



The protoplast can absorb the last traces of oxygen, but below a certain 

 dilution the amount absorbed is insufficient to supply all requirements. In 

 the higher plants this point is reached only when the percentage of oxygen 

 falls below 5 to 8 per cent. 5 , and hence they are able to respire normally 

 on the summits of the highest mountains. If, however, the absorption 

 is rendered more difficult, as, for example, by smearing them with fat, the 



* Elfving, Einwirkung d. Lichts auf Pilze, 1890, p. 125. 



2 Godlewski, Anzeig. d. Akad. d. Wiss. zu Krakau, Juli, 1897. 



3 Pfeffer, Unters. a. d. Bot. Inst. z. Tubingen, 1885, Bd. I, p. 684; Oxydationsvorgange, i! 

 pp. 449 , 500; Celakovsky, Flora, Erg.-bd., 1892, pp. 194, 209, 226. 



4 Reinke, Bot. Zeitung, 1883, p. 95; Pringsheim, Sitzungsb. d. Berl. Akad., 1887, p. 772. 



5 Cf. Stich, Flora, 1891, p.' i, and the literature there quoted. 



