252 SEAWEEDS 
It is most convenient to consider the Nostocacee 
under two families, viz. the Heterocystew and the 
Homocystec. 
Heterocystece. 
General Characters.—The cells of the trichome are 
differentiated into vegetative cells and into Aeterocysts, 
or cells incapable of farther development. The 
elongation of the trichome is by transverse division 
of all the cells (tribe Nostocee), or of a meristematic 
group of cells (tribe Rivulariew). There is true 
branching only in the tribe of Strosiphonicw, 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 Nostocew, where they all appear 
to be very much alike; differences appear in those 
forms classified under Scytonemew, and are most 
marked of allin the tribe of Rivularicw. 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 
Nostocacece) may be mucilaginous, gelatinous, or car- 
tilaginous in consistence, and there is considerable 
variety in its form and other characters, of service in 
