THE BLUE ALG.E 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 f avoral^le 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 Chroococcus may be taken as typical of the Class 

 Cyanophycese 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 inasked by phycocyanin, arid 

 by consisting of single cells or cell-colonies of various form, reproducing 

 only by fission, although the colonies sometimes midtiply 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 algse 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 alga3 have an especial 

 importance as being the chief cause of offensive odors which 

 ' Hor"mo-gon'i-um < Gr. hormos, a chain; gonos, offspring. 



