FRESH-WATER ALG. 211 
The filaments of all the species of Oscillatoria 
are elastic, simple, exceedingly minute, and mathe- 
matically straight. They are distinguished by 
close parallel rings or transverse markings easily 
separating from each other. The motion of oscil- 
lation, for which all the species are distinguished, 
is in some remarkably vivid, and would favour the 
supposition that they are animals and not plants, 
were it not that their other characteristics are 
peculiarly those of vegetables. The filaments con- 
tinually move from right to left, or from left to 
right, but in a very irregular manner, some going 
in one direction and others in another ; some being 
at rest while others are in motion. This lateral 
oscillation has been attributed to various causes. 
The majority of naturalists, inclining to the opi- 
nion that it is mechanical and not voluntary, have 
ascribed it to rapidity of growth, which, in such 
simple plants, is excessive; to the molecular action 
of light, or to the agitation, by hidden causes, of 
the water in which the filaments are immersed for 
inspection. But none of these suppositions afford 
a satisfactory explanation, as Captain Carmichael 
ascertained by the following simple contrivance : 
He placed a small portion of the stratum of a 
species of Oscillatoria, composed of a great many 
individuals united together, in a watch-glass filled 
with water, and coveted it with a thin plate of 
