236 
WILLIAM J. ROBBINS 
of starch made in these three types of water, distinct evidences of 
toxicity were noted from the laboratory distilled water. The my- 
celium in this distilled water was knotty in appearance and in 
small tufts. The mycelium in redistilled water or in distilled water 
treated with carbon black was fluffy in appearance and in a connected 
mat. The apparent toxicity of the distilled water, due perhaps to the 
presence of iron noted above, led to the use of either redistilled water 
or distilled water treated with carbon black throughout the experi- 
ments. 
On comparing the digestion of starch by Penicillium camemhertii 
in the three types of water, it was found that this was generally the 
most rapid in redistilled water, and in most cases slowest in distilled 
water. The presentation of this phase of the subject is reserved for a 
future paper. 
Sufficient carbon black for the entire investigation was purified at 
one time by washing it during a period of a week with nine changes of 
distilled water and two of redistilled water. The carbon black when 
used contained approximately 72 percent of water. The amount 
used per 4 liters of water was therefore equivalent to about 25 grams 
of dry material. 
Analytical Methods 
In the experiments with nutrient cultures the rate of starch 
digestion was determined by finding the number of days required by 
the fungus to digest completely the starch in the culture medium. 
Complete digestion was determined by the Katz (1898) method. 
This consists in brief of the removal, daily, of a drop of the culture 
fluid under antiseptic conditions and the determination of the presence 
of starch by the use of iodine. When the drop removed shows no 
coloration with iodine, digestion is considered complete. 
In the experiments with single salts it was deemed preferable to 
determine the amount of starch digested at a given interval from the 
time of inoculation. This procedure is more accurate and less tedious 
than the Katz method, and has the advantage of permitting the 
determination of the starch digestion when the fungous mycelia of all 
the cultures of a given series are of the same age. 
Difficulty was experienced, however, in finding a method suitable 
for determining how much of the starch originally present in a culture 
solution had been digested by a fungus growing therein. It was con- 
