THALLOPHYTES 
often called arthrospores, but they are not spores in the same sense as 
are those which characterize higher plants. In its life history, there- 
fore, Nostoc displays three kinds of cells: vegetative cells, heterocysts, 
and resting cells. 
A common form very closely related to Nostoc is Anabaena, whose name ought 
to be familiar, but whose separation from Nostoc need not be attempted by the 
elementary student. 
Rivularia. This form may be taken to represent the extreme differentiation 
of a colony. It is a compact, filamentous plant, like Oscillatoria; but the basal 
cell of the filament is a heterocyst, and the apex of the filament tapers into a very 
slender, whiplike extension (fig. 9). In this case the filament has a distinct 
base and apex. 
Tolypothrix. This plant serves to illustrate what is called false branching. 
It is a filament with distributed heterocysts, and, therefore, composed of several 
FIGS. 10, ii. False branching: 10, Tolypothrix, showing false branching by a 
hormogonium pushing past a heterocyst; n, Scytonema, showing false branching by the 
pushing outward of two abutting cells of a hormogonium, each of which continues 
division. 
hormogonia. In some cases the end of a hormogonium pushes past a heterocyst 
and continues division, giving the appearance of a lateral branch (fig. 10). In 
other cases, as in Scytonema, a hormogonium may continue to increase in length 
without breaking away from the heterocysts, and the pressure results in push- 
ing some two abutting cells outward, each of these two cells then being free to 
continue the development of a filament (fig. n). 
