1914] 
SCHRAMM—PURE CULTURE METHODS IN THE ALG 29 
along by the lengthening filaments. Except in rare cases, 
nothing is to be gained by this procedure in the alge. The 
task of isolating pure cultures of alge, therefore, becomes an 
individual problem for almost every species as it necessitates 
at once the determination of the period in the life history of 
any form at which the cells are free from bacteria or at which 
time the bacteria can be removed by one means or another. 
Having found a stage in which the alga is bacteria-free, it is 
of importance next to be able to bring about this stage 
more or less at will in order that the alga may be utilized when 
available. To obtain the above preliminary information, noth- 
ing is more serviceable than the usual plating method on a 
suitable medium. 
The Medium.—The requirements of a suitable solid medium 
for algal isolating purposes are, that it remain liquid down to a 
temperature at which delicate algal cells are not injured; that 
it be suitable for the growth of alge, and as unfavorable as 
possible for the growth of bacteria and fungi. For this purpose 
nothing was found so serviceable as the following, the mineral in- 
gredients being in the proportions recommended by Moore (17): 
Agar 10.0 grams 
NH; NO; 0.5 gram 
MgSO.. 7H;0 0.2 gram 
К.НРО, 0.2 gram 
CaCl: 0.1 gram 
FeSO. trace 
Dist. НО 1000 сс. 
The agar should be carefully washed, first in а stream of tap 
water and then in distilled water, as pointed out by Richter (20). 
An agar so prepared will remain liquid down to about 34.5- 
35°C., and experience has shown that even the most delicate 
algal cells are uninjured by the short exposure to this temper- 
ature necessary in the plating process. From six to eight cc. 
of agar in a Petri dish eight em. in diameter is а suitable quan- 
tity with which to plate. Larger quantities so thicken the layer 
of agar in the dish that the higher powers of the microscope, 
with their objectives of short focal length, cannot be used in 
locating small developing colonies. 
