252 SEAWEEDS 



It is most convenient to consider the Nostocaccce 

 under two families, viz. the Heterocystecc and the 

 Homocystcce. 



Heterocystcm. 



General Characters. The cells of the trichome are 

 differentiated into vegetative cells and into hctcrocysts, 

 or cells incapable of farther development. The 

 elongation of the trichome is by transverse division 

 of all the cells (tribe Nostoceoe), or of a meristematic 

 group of cells (tribe Eivulariece). There is true 

 branching only in the tribe of Sirosiphonicce, effected 

 by the division of cells parallel to the axis. Hormo- 

 gonia, and spores endowed with the power of resting 

 and of thus preserving the plant during unfavourable 

 seasons and periods of dryness in particular, are the 

 characteristic modes of reproduction. 



The Thallus. The vegetative cells of the trichome 

 vary least in the tribe Nostoccce, where they all appear 

 to be very much alike ; differences appear in those 

 forms classified under Scytonemccc, and are most 

 marked of all in the tribe of Rimdaricce. The cells at 

 the tip of a filament are generally shorter than at the 

 base, where they attain an elongate cylindrical form. 

 Cell-division takes place ordinarily when the cell 

 attains its maximum length, but when growth is 

 active the divisions succeed each other before that 

 is reached and while the cells are still short. The 

 envelope of the trichome (called the sheath in all the 

 Nostocacecv) may be mucilaginous, gelatinous, or car- 

 tilaginous in consistence, and there is considerable 

 variety in its form and other characters, of service in 



