FRESH-WATER ALG. 213 
are developed. Another strange fact in the 
economy of these very singular and anomalous 
plants is the extremely limited term of their ex- 
istence. Their cycle of life is often completed in 
three or four days. The community of individuals 
associated together in one patch or stratum live 
for several months; but the individuals them- 
selves die off, and are succeeded by others with a 
rapidity truly marvellous, The remains of the dead 
filaments form the bases of the living ones, and 
thus they go on increasing in depth and breadth 
until they often cover the whole bed of a stream- 
let. This peculiarity connects them with the 
coral-zoophytes, and supplies another link between 
the animal and vegetable kingdoms. Mr. Sorby 
has recently made some valuable observations in 
the spectrum analysis of the Oscillatorias. Ehren- 
berg called the lowest organizations, belonging 
to the fresh-water alge, ‘the milky-way of the 
vegetable kingdom;’ and it is an interesting reflec- 
tion that the same method which has yielded such 
wonderful results when applied to the nebule of 
telescopic research, is promising to be equally 
successful when applied to the nebule of micro- 
scopic research. By examining the spectrum of 
a coloured solution obtained from certain species 
of Oscillatoria, he has found three perfectly dis- 
tinct colouring matters, which also occur in lichens, 
