640 The American Naturalist. [ August, 
Even the slightly varying acidity of steamed slices from differ- 
ent potato tubers may exert a marked effect on the growth of 
certain sensitive organisms. On this account some bacterio- 
logists have advised discarding the potato altogether. I have 
myself found the potato a very useful substratum for the growth 
of both fungi and bacteria. All comparative tests on potato 
ought, however, to be made on cylinders or slices cut from the 
same tuber, and in every case the reaction, acid, neutral, or 
alkaline, should be carefully recorded. The behavior of the 
organism on a variety of tubers should also be determined, 
before deciding that it is something new. It has been thought: 
by some that the best nutrient substance for a parasite must be, 
unquestionably, the juices of the host plant but this does not 
follow since there are all grades of parasitism, and even if it 
did, there are several chances for error in its employment, 
e. g. the nutrient juices are usually sterilized by steam heat 
and this may cause a number of chemical changes resulting in 
a compound very different from the living plant and entirely 
unsatisfactory as a culture medium, as many have learned by 
experience. Again, for some particular reason, even the juices 
of the plant when sterilized at ordinary temperatures by filtra- 
tion, may be less well adapted to the needs of the parasite than 
well made beef bouillon or ordinary nutrientagar. In general, 
the standard culture media of bacteriology should be tried first. 
Some bacteria can be cultivated only on special media or at 
special temperatures, or under unusual conditions. Bacillus 
subtilis will only grow in the presence of free oxygen ; Bacillus 
‘amylobacter, B. tetani, and B. carbonis will only grow in the 
absence of oxygen. Winogradsky states that his nitrifying 
organism obtained from European soils will not grow in the 
ordinary culture media and thrives best in solutions of in- 
organic substances, and on silicate jelly. Bacterium tuberculosis 
can be cultivated only in bouillon and on blood serum and 
‘nutrient glycerine agar, and at temperatures above 30°C. Bac- 
terium influenze also flourishes at blood heat and can only be 
grown, it is said, in the presence of red blood corpuscles or in 
media containing yolk of eggs; other organisms have thus far 
‘refused to be cultivated at any temperature or on any artificial 
medium, e. g. Bacterium lepre and B. syphilitis. Some bacteria 
