RIVULARIA. 249 



conclusion, that Rivularia echinulata and Gloeotrichia Pisum are 

 probably identical. These plants like most of the algse multiply 

 very rapidly by means of spores and by hormogonia. One fila- 

 ment gives rise to five or more hormogons, each of which in 

 the course of several days gives rise to as many more thalli ; 

 thus in a comparatively short time millions of thalli may be 

 produced. 



The following two diagnoses may be preserved as matters of 

 historic interest, and for reference : 



RIVULARIA ECHINULATA, Smith. 



Thalli globose, minute, dark olive-green or brownish. 

 Trichomes fastigiate, attenuated upwards to the apex, 

 closely cohering, articulate ; heterocysts basal, globose ; 

 sheaths very narrow, inconspicuous. 



Diameter of trichomes, at base, about 7 ^ ; 250 yw, more 

 or less, long. 



Syn. Chaetophora punctiformis, Kg. ; Echindla articulata, Eng. 

 Fla. ; Conferva echinata, Eng. Bot. ; Conferva echinulata, 

 Gray, etc. 



Plate CLXXIX, fig. 4, a thallus from the surface of lake, 

 Minnesota, collected by J. C. Arthur, noted above, flattened 

 under cover glass of slide ; fig. 5, natural size of thalli. 



RIVULARIA DURA, Kg. 



Thalli nearly globose, about the size of mustard seeds, 

 sometimes larger, dark olive green, brownish or n^rly 

 black. Trichomes aeruginous, gradually tapering from the 

 base to the hyaline apex, or sometimes variable in the same 

 thallus ; some apparently withered, others more swollen, 

 some inarticulate, others torulose ; all with distinct sheaths. 

 Lower cells as long as broad or nearly so, upper ones longer, 

 all with cytioplasm granulated ; sheaths colorless or yel- 

 lowish ; heterocysts rounded. 



Diameter of trichomes, 8-9 ^ at the base. 



Syn. Limnactis dura, Kg. ; Rivularia radians, var. dura, Kirch. ; 

 Dasyactis mollis, Wood. 



Attached to aquatic plants in ponds. 



Plate CLXXIX, fig. 1, transverse section of a large 

 thallus ; fig. 2, a section showing the development of spores ; 

 fig. 3, a section with withered, or fading trichomes. 



