BIOGRAPHICAL MEMORANDA 



- either adhering closely to the surface of the medium, or forming more 

 or less pronounced and raised folds - can often be greatly modified by 

 factors that are difficult to control, then it is no longer surprising that 

 different results are so often obtained in duplicate cultures under pre- 

 sumably identical conditions. But even if such cultures were to show 

 satisfactory agreement it is still impossible to specify sharply the con- 

 ditions responsible for a particular direction of the metabolic pro- 

 cesses. If, for example, a particular situation leads to the formation of 

 large amounts of citric acid, it is impossible to decide which of the 

 cells in the mycelium, developing under heterogeneous conditions, 

 have been responsible for the acid production. And even if it were 

 possible to determine which cells have produced the acid, and to 

 specify the optimum conditions for this process, the result would still 

 be of only limited significance, because it would be erroneous to con- 

 clude that any cell of the organism would display the metabolic activ- 

 ities characteristic of the acid production under these same optimum 

 conditions. One can find many examples in the literature showing that 

 the metabolism of mould cells is greatly dependent on the conditions 

 under which the cells were grown, and we shall presently add further 

 proof for this statement. Now it is obvious that these conditions are 

 quite heterogeneous for the cells developing in a mycelial mat on a 

 liquid medium. In summary we may therefore conclude that one 

 has to reckon with the presence in such a mat of cells of heteroge- 

 neous origin, exposed to heterogeneous conditions. It will then be clear 

 that one can obtain at best a very restricted picture of the metabolic 

 activities of moulds with the aid of the usual stationary cultures. 

 Consequently a deeper penetration into the problems of this metab- 

 olism is possible only if it is investigated with cell material of homoge- 

 neous composition, studied under homogeneous conditions' (p. 70). 



This, of course, raised the question how such cell material of homo- 

 geneous composition could be procured. At the outset it seemed pos- 

 sible that this might be accomplished by growing the mould so that it 

 would develop submerged in a culture liquid rather than on top of it. 

 This would necessitate aeration of the culture in order to supply the 

 cells with an adequate amount of oxygen. After testing a number of 

 different arrangements the most satisfactory results were obtained by 

 inoculating a liquid medium with mould spores, and placing the 

 culture on a shaking apparatus, thus maintaining the contents in a 



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