THE BLUE ALG 475 
breaks into short lengths each made up of a comparatively 
small number of cells. These groups are termed hormogonia.' 
While the cell-row is dividing into hormogonia the gelatinous 
envelope is becoming fluid, and as the hormogonia separate 
they are observed to take on a swaying, worm-like movement 
which enables them soon to pass into the surrounding water 
and travel in various directions. In a little while they come 
- to rest, secrete a new gelatinous envelope, and by repeated 
fission of the cells develop a new colony. On the approach of 
adverse conditions, such as those of winter, certain of the 
cells protect themselves by a dense wall, store up food in 
the cytoplasm, and become brownish. In this condition 
they are very resistant of cold and tolerant of drying, and 
are thus able to survive unharmed under conditions which 
destroy the. other cells. These resistant cells are called 
resting spores. When the other members of the colony perish 
and the mucilaginous envelope dissolves, these spores are set 
free; and on the return of favorable conditions they germinate 
by a swelling of the protoplasm which ruptures an outer 
dense layer of the wall. Covered then by a thin, inner layer 
the protoplasm elongates, as shown in Fig. 306, B, C, D, and 
by repeated fission together with a copious secretion of jelly 
a new “‘fallen star”’ colony is started. 
Nostoc and Chrodcoccus may be taken as typical of the Class 
Cyanophycee the members of which occur abundantly in salt or 
in fresh water or on surfaces frequently wet, and are characterized 
by having their chlorophyll more or less masked by phycocyanin, and 
by consisting of single cells or ceil-colonies of various form, reproducing 
only by fission, although the colonies sometimes multiply through 
hormogonia or resting spores. 
The lower members of the class have about the simplest 
organization known. They are doubtless as much like the 
earliest organisms which appeared upon the earth as any 
creatures now living. It is interesting and perhaps signifi- 
cant, that the alge previously referred to as thriving in the 
scalding water of hot springs, belong to this class. From an 
economic point of view the blue-green algz have an especial 
importance as being the chief cause of offensive odors which 
1 Hor’’mo-gon’i-um < Gr. hormos, a chain; gonos, offspring. 
