[Vor. 1 
42 ANNALS OF THE MISSOURI BOTANICAL GARDEN 
leaving the latter behind, it is fairly certain that the hormo- 
gonium is originally free from bacteria, but becomes contam- 
inated in passing through the older portion of the empty sheath 
and out of its terminal opening, both of which are more or less 
infected with bacteria. The persistence with which the bacteria 
cling to the hormogonium of Oscillatoria, once having infected 
it, is clearly shown by cultures on agar surfaces. Although 
a single hormogonium may have moved as much as two inches 
away from its parent filament, creeping all the while over a 
sterile agar surface, the hormogonium will be found covered 
with bacteria, and the path over which it moved will be clearly 
indicated by a continuous, linear colony of bacteria. With 
the use of silicic acid jelly, however, the multiplication of the 
bacteria is reduced to such an extent that, after a time, hormo- 
gonia escape uncontaminated, and begin the development. of 
pure colonies. Transfers from these, however, grow very 
slowly and in most cases eventually die. It seems probable, 
when Oscillatoria and Microcoleus have been completely sepa- 
rated from the invariably present bacteria, that the media 
which were favorable in the presence of the bacteria, become 
unfavorable in their absence. Further work will be necessary 
to grow these forms successfully after they have been isolated 
in pure form. The silicic acid jelly method was also attempted 
with the above mentioned heterocystic forms; however, up 
to the present time, no successful isolations have been made. 
Discussion 
It is apparent that the technique involved in the isolations 
just referred to depends entirely on mechanical separation 
of one kind or another. This method is reasonably efficient 
in those species in which zoóspores or other free endogenous 
spores are readily obtainable, or in which vegetative cells are 
either free from bacteria or can be rendered so by mechanical 
means. It is true that even in some species forming free 
endogenous spores, the above methods have not yielded pure 
cultures, as, for instance, in Vaucheria, Oedogonium, and Spiro- 
gyra. In these cases, however, it should be pointed out that 
it is not the isolation technique which is at fault but rather 
