248 FRESH-WATER ALG-JE OF THE UNITED STATES. 



fig. 1, a young form of Gloeotrichia; fig. 2, half of a trans- 

 verse section of a smaller thallus ; spores formed, but sheath 

 not yet evident ; fig. 3, a half of a similar section of a 

 thallus arrested in its growth and fading. 





 Genus 85, RIVULABIA. 



i 



Filaments radiately arranged, agglutinated by a more or less 

 firm mucilage and unitedly forming hemispherical, or bladdery 

 well-defined forms. Heterocysts basal. 



Forms of this genus, in my earlier experience, were supposed 

 to be frequent, but now I question whether any of them can be 

 separated from the genus Gloeotrichia. The form most familiarly 

 known as Rivularia dura, Kg. ; with such synonyms as Limnactis 

 dura, Kg. ; Rivularia radians, var. dura, Kirch. ; Limnactis flagel- 

 lifera, Kg. ; Limnactis minutula, Kg. ; and others, is often very 

 abundant in ponds, attached to water plants ; it is a true Rivu- 

 laria, in accord with the diagnosis of the genus ; but gathered at 

 a later period of the year the spores, and the sheaths, evidences 

 of the genus Gloeotrichia, develop themselves. Of the thousands 

 which may be quickly gathered many are found to have devel- 

 oped the characteristic spores and sheaths ; while others retain 

 their simple form, and yet others have the trichonies shrunken 

 or partially withered, showing signs of a state of decay. It has 

 been shown how young forms of G. natans, gradually develop the 

 mature trichomes ; Plate CLXXVIII, 4-7 ; figs. 8-12, etc. In a 

 collection of many thalli of Rivularia dura all possible stages of 

 development may be found in proper season. 



Judging simply by personal observations on living plants, and 

 by analogy, all Rivularia should be transferred to Gloeotrichia. 



There is one form, however, of which we have not seen living 

 plants ; apparently the same, which was found in England, and 

 reported in the English Botany as early as 1804, named by 

 Smith, Conferva echimdata, (indexed as Rivularia echinulata). 

 Since then it has been found repeatedly in England and on the 

 Continent. Prof. Cohn reports it as Rivularia fluitans, covering a 

 sluggish river for miles in extent. 



Prof. J. C. Arthur, of the Iowa Agricultural College, reported 

 a. similar, or the same plant, found as a scum of large extent on 

 "Western lakes. 



Bornet and Flahault, of France, made a careful examination 

 of this form of Rivularia, but unfortunately had not living 

 plants. Judging by the specimens they had, they reached the 



