10 STEINBERG: GROWTH OF ASPERGILLUS NIGER 
sider the minimum zinc concentration resulting in maximum yield; 
we find that the numerical value of this concentration increased 
as the experiments progressed, or, as previously stated, that in 
this respect also the sensitiveness of the organisms to the action 
of zinc has decreased. : 
Inspection of TEXT FIG. 4 suggests that to some extent at least 
the experimental deviations should be laid to an uncontrolled vari- 
ation of the zinc concentration, or of other substances acting in the 
same direction. There is noticeable, for example, a decrease in 
the action of zinc (0.01 mg. Zn/L) as a rule when the simultaneous 
supposedly zinc-free cultures yield a weight below average. Sim- 
ilarly, the opposite-increased effectiveness of the action of zinc 
when the control cultures give a yield above average, is also 
evident on inspection of the curves for the twenty-eighth to thirty- 
first week inclusive. 
In casting about for the causes underlying this gradual change 
in the strains, the assumption was made, that it was due to the non- 
addition of inorganic salts to the peptone-sucrose agar used for 
the stock cultures. This at least occurred to me as the simplest 
and most obvious explanation. 
In order to test this assumption subcultures were made from 
the peptone-sucrose agar to other agar compositions containing 
various mineral salts. After three generations on the latter, they 
were used as usual for inoculating the liquid Pfeffer medium and 
the yields obtained compared with those obtained through the 
use of the standard peptone-sucrose spores. Thus transfers from’ 
the seventh generation cultures of the W and Y organisms cul- 
tured on the peptone-sucrose agar were made to a medium of the 
following composition: 
Water oii ee re a 1000.0 gram 
BUICTOOE 6 PSE ae ee eee Oss SOD) 5°" 
Peeing ese ce yee Pen Ce eyes pa eas 
INTENO Reh es a 10.654 
Baker's eres bes a a se 2 ae bate 
“Aneiyeed ) MysOce Ns. . ei ee Pega 
MCU SOn Ee Or ee oe trace 
Cultures on this medium are referred to as 7W and 7Y. In 
appearance they correspond quite closely to that of a zinc culture. 
