1914] 
SCHRAMM—PURE CULTURE METHODS IN THE ALG/E 81 
colonies varies greatly with the species, from one to three or 
four weeks usually being required, depending upon the particular 
form. In most cases it is not possible to wait until the algal 
colonies ean be seen macroscopically because spreading bacterial 
and fungal colonies usually encroach on the former to such 
an extent that a pure transfer is no longer possible. It becomes 
necessary, therefore, to look the plates over from time to time 
with the compound microscope in order to locate algal colonies in 
very earlystages of development. For this purpose а 12 mm. ob- 
jective is extremely serviceable, as its focal length is of sufficient 
magnitude to enable one to use it through the agar layer and 
glass bottom -of а Petri dish and at the same time obtain a 
magnification considerably greater than that afforded by the 
ordinary low-power objective. The colonies located are conven- 
iently marked by placing a small ink dot directly opposite them 
on the bottom of the Petri dish. Transfers should be made to 
agar slants by means of a minute platinum-foil spatula with which 
the agar directly over the ink dot can be neatly dissected out 
and transferred to the slant. It is not possible, in most cases, to 
make successful transfers with a platinum needle because the 
algal colony is usually composed of firmly cohering cells and, 
even in repeated attempts, not a single individual will adhere to 
the needle. Since many of the colonies are in the deeper strata, 
it is well to spread out the transferred agar fragment in a 
thin sheet in order to expose the contained algal cells directly 
to the air. Unless this is done, subsequent development in 
the slant may be extremely slow. Although bacteria grow 
slowly on this synthetic agar, their development is usually 
sufficient in a week to indicate whether the transfer has been 
successful or not. The purity of the culture may be further 
tested by making transfers to media more suitable for bacterial 
growth. 
With this brief preliminary consideration of some of the more 
general phases of pure culture technique in the alge, the isola- 
tion of single species will now be considered and attention called 
to the special problems and the technique involved in their 
isolation. 
