88 MIDLAND NATURALIST. 
that the disappeared forms will rather suddenly come back, often in 
better condition than when first found in the field, or if the plants 
do not appear, others equally or more valuable will develop, for 
which we have sought with as much care. Algae are never found 
in their native habitat in pure culture. The disappearance of one 
specimen is often but the reason of the appearance of another that 
only waited for its predecessor to go in order to come upon the 
scene itself. When the same algal forms reappear it is but a sign 
the resting stages of the reproductive organs had been slowly 
adapting themselves to an environment in which they finally found 
it possible to germinate and thrive. 
The contents of an aquarium need only be thrown out when 
complete organic decomposition has taken place which is indicated 
by the presence of bacteria, often by a purple-violet coloration of the 
water and always by an over-powering disagreeable odor of decay, 
Even after this, however, desirable plants in form of Myxophyceae 
especially Oscillatoria have subsequently been known to develop. 
Another instance in which it is advisable to pour out the contents 
of the jar is when a small form of Oscillatoria appears. This is 
easily recognized under the high power of the microscope, but even 
as a type of Oscillatoria it is not valuable, as it is too small to serve 
for demonstration. The cells and contents are better brought out 
by larger forms. Moreover the plant prevents the development of 
all other desirable plants except other Myxophyceae such as 
Glaeocapsa, Chroococcus, Anabaena, Nostoc, Tolypothrix, Cylin- 
drospermum and Rivularia. Whenever this small Oscillatoria 
obtains possession of a jar it destroys everything else either by 
crowding it out or choking it to death or actually posioning it, by 
means of some evil smelling compound partly soluble in the water 
in which the plant is found. Even Cladophora which is one of the 
most hardy of the algae and which will stand continuous changes 
of water quite well, and will remain several years unchanged in a 
small bottle without being affected noticeably, will disappear quick- 
ly under the influence of this small dark blue Oscillatoria. 
Complete removal of this enemy of the aquarium is only effect- 
ed after very thorough disinfection of vessels with formaldehyde, 
or another equally energetic disinfectant. The Oscillatoria does not 
seem to prevent the growth or reappearance of other Myxophyceae 
(Cyanophyceae or Blue Green Algae) and a good culture of any of 
these not be disposed of to prevent the ravages of the enemy. 
