434 



PROTOPHYTA 



the colony is usually invested, but in most species also a separate delicate 

 membranous sheath to each individual, often of a yellowish colour, which 

 ultimately becomes open at the apex. In Microchaste diplosiphon 

 (Gomont, Bull. Soc. Bot. France, 1885, p. 209) this sheath is double. 

 The common gelatinous envelope of the 

 colony sometimes becomes at length calca- 

 reous. In other species, which float on the 

 surface of the water, the development of 

 mucilage is much feebler. 



The filaments of the Rivulariacese branch 

 in a peculiar way. A small portion of the 

 green tract between the basilar cell and the ter- 

 minal bristle becomes rounded off, loses its 

 endochrome, and forms a heterocyst resembling 

 the basilar cell. This heterocyst now assumes 

 the part of a basilar cell ; the terminal por- 

 tion of the filament beyond it detaches itself, 

 and applies itself laterally to the heterocyst, 



FIG. ^.Rivularia^olyotis Hauck. a, invested in its gelatinous envelope (natural size). 

 6, vertical section (x I2 ). c, filament ( x 150). (After Hauck.) 



forming a ' pseudo-ramulus ' or false branch ; the original filament then 

 develops a new apical portion in a direct line above the heterocyst. 

 Heterocysts have not yet been observed in all the genera ; they some- 

 times occur interstitially in the filament without giving rise to a pseudo- 

 ramulus ; their function is obscure. The terminal hyaline bristle is of 



