ENERGY SUPPLY OF THE CELL 157 



Changes of type might be brought about by the fact 

 that the same organism has different minimum tem- 

 peratures for different substrates. Monilia sitophila 

 grows well with citric acid at higher temperatures, but 

 not at 15°C. or lower, where it still thrives with dextrose 

 (Went, 1901). Similarly, Penicillium, according to 

 Thiele (1896), cannot grow with dextrose if the tempera- 

 ture rises above 32°C., but can still live there with 

 glycerol, while at lower temperatures, sugar is by far 

 the better food. For Aspergillus, the same author 

 found the minimum temperature to be 6°C.-8°C. if 

 grown on dextrose, but it was 10°C.-12°C. if grown on 

 formic acid salts. 



(c) INFLUENCE OF CONCENTRATION OF SUBSTRATE 



A rather unexpected experience is the more recent 

 observation that the same substrate at different con- 

 centrations yields different products, or at least different 

 ratios of the products. The first experiment carried 

 out on a large scale is that of Mendel (1911), who studied 

 the products formed by various gas producing bacteria 

 from dextrose, maltose, lactose and sucrose in concen- 



CO 



trations from 1 to 20%. He found that the -Tr~ ratio 



-tl2 



was in some cases very distinctly influenced by the 

 concentration. 



These data, interesting as they are, cannot be taken 

 at full value because the gas ratios CO2 : H2 contradict all 

 other experiences. The hydrogen and carbon dioxide 

 headings of the tables probably became confused in 

 printing. But whatever it be, there is some influence 

 of concentration upon the gas ratio in some instances, 

 and upon the ratio between volatile and non-volatile 

 acids as well, which deserve attention. 



