THALLOPHYTES 9 



often called arlhros 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, n. — False branching: 10, Tolypothrix, showing false branching by a 

 hormogonium pushing past a heterocyst; 11, Scytoncma, showing false branching by t le 

 pushing outward of two abutting cells of a hormogonium, each of which contini es 

 division. 



hormogonia. In some cases the end of a hormogonium pushes past a heteroc; st 

 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 pu:h- 

 ing some two abutting cells outward, each of these two cells then being free to 

 continue the development of a filament (fig. 11). 



