THALLOPHYTES 



often called arthros pores, 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. u). 



