OTANOPHTGE^. 317 



composed of rounded cells loosely united into a filament and 

 generally imbedded in jelly (Fig. 148, A) ; they frequently 

 form large masses, united by the glutinous jelly. At inter- 

 vals in the filaments there are larger clear cells — the hetero- 

 cysts — which appear from analogy to be reproductive bodies, 

 although nothing is positively known as to their function. 

 The usual mode of reproduction is by the simple fission of 

 the cells. New masses or colonies are formed by the break- 

 ing up of the old filaments into pieces composed of a few 

 cells, which then become endowed with a power of motion 

 which consists of a slow bending from side to side with a 

 forward movement at the same time. Each moving fila- 

 ment, when it comes to rest, may become the centre of a 

 new colony, which arises from it by fission. 



Four genera and twenty or more species are known in the United 

 States. The principal genus 

 is Nostoe (Fig. 148, A); its ^ 



species form jelly-like masses "^ 



from the size of a pin-head to 

 several inches in diameter in 

 ponds and streams, adhering 

 to sticks and twigs, and on wet pig. \^,—a, a filament of a Nostoc, with a 



Tnclr-s or wet ffround • thev '^■'S^ heterocyet ; B, end of filament of Os- 

 rocKS or wei grouuu , biiey ^j,^^^^^_ ^ 300.-After Prantl. 



even grow inside of other 



plants — e.g.,Anthoceros Imvis — and, according to the present view, con- 

 stitute the so-called gonidia of certain lichens. 



284.— Order Osoillatoriacese. The filaments in this or- 

 der are composed of more closely cohering cells than in the 

 previous one ; the cells unite by broad surfaces to form a 

 rigid, cylindrical, straight or slightly curved filament (Fig. 

 148, B). They form dark-green, loose, or felted masses in 

 water or on wet earth, and are remarkable for the peculiar 

 oscillating movements of their filaments. No other method 

 of reproduction than by fission is known. 



The principal genus is Oscillatoria, of which we have at least half 

 a dozen species. 



285.— Order Rivularia-cese. The filaments in this order 

 present a greater differentiation than in any of the preced- 

 ing ; they are usually arranged in a radiating manner, and 

 imbedded in a common jelly, so as to form small rounded 



