CYANOPHYCE^. 91 



chain are in contact, being connected by strings of proto- 

 plasm passing through perforations in the transverse walls ; 

 in fact we have continuity of protoplasm exhibited contem- 

 poraneously with the most primordial attempt to form 

 multicellular plants. During the vegetative phase all the 

 cells are morphologically alike, and add to the length of the 

 filament by repeated division. The two ends of a filament 

 are alike, that is, there is no differentiation into base and 

 apex. During the vegetative development of a filament 

 certain of the ordinary cells increase in size, become in- 

 capable of cell-division, and their contents are replaced by 

 colourless cell-sap. Such cells are termed heterocysts, and 

 may be either terminal or intercalary ; their function is 

 unknown. The filaments increase considerably in length by 

 cell-division, and become much contorted and more or less 

 interwoven ; the gelatinous sheath in which they are involved 

 usually becomes confluent, and forms an outer gelatinous 

 mass in which the filaments are imbedded. Reproduction 

 is effected by hormogones, that is, portions of a filament 

 composed of strings of four to eight cells break away, 

 escape from the gelatinous envelope, and develop into 

 new individuals ; or certain of the cells of a filament are 

 transformed into spores. 



In the species of Rivularia and Calothrix the filaments 

 do not consist of elliptical or sub-spherical cells or pseudo- 

 cysts as in Nostoc, but are without constrictions, the thread 

 being cut up into superposed cells by delicate transverse 

 septa, thus approaching nearer to a filament composed of 

 true cells. Such filaments also show an advance on Nostoc 

 in having the two ends differentiated ; the basal end 

 terminating in a large, colourless, basilar cell, wliile the apex 

 is attenuated into a long, tapering, colourless hair. 



