44 FRESH-WATER ALG^E OF THE UNITED STATES. 



frond into a periderm. The filaments commonly radiate from the centre to the 

 circumference ; sometimes, especially in the softer fronds, they are simply parallel 

 with one another. The sheaths vary in their breadth, firmness, and distinctness. 

 These little plants grow chiefly in the water ; some species are said to live in 

 the air in exceedingly damp places, but I have not as yet met with any such. 

 They appear to prefer cold climates, although I have received specimens from. 

 South Carolina. With us, I have only found them in the late autumn and winter 

 months. As to their life-history very little appears to be known; I have not been 

 able to make any observations myself upon this point, nor to obtain access to the 

 papers 1 by De Bary, almost the only sources of such information, and therefore 

 pass by the subject. 



Genus NOSTOCHOPSIS, WOOD. 



Trichomata ramosa cum cellulis perdurantibus aut in lateribus sessilibus aut in ramuloruin brevissi- 

 inorum apicibus dispositis. Vaginae nullse. Thallus definitus. 



Syn. Nustochopsis, WOOD, Prodromus, Proc. Amer. Philos. Soc., 18G9. 



Thallus definite ; filament branched ; heterocysts sessile upon the sides of the filaments, or raised 

 upon the apices of short branches ; sheaths none. 



Remarks. The curious plant upon which this genus is founded has the habit 

 of a nostoc. The outer portion of the frond is condensed, so as to give the appear- 

 ance of a periderm ; but there is, in reality, no true periderm. The consistence 

 of the thallus is that of a firm, gelatinous mass. The trichomata or filaments 

 radiate from the inner part of the frond towards the outer surface, but many of 

 them take their origin in the outer portions of the thallus. In most places they 

 are distinctly articulated, and, indeed, the joints being swollen and moniliform, in 

 some parts they almost seem to be composed of globular cells, resembling some- 

 what the filaments of a nostoc ; on the other hand, in certain portions they are not 

 at all articulated, and this for long distances. No sheaths are anywhere visible. 

 The heterocysts are, strangely enough, never placed in the continuity of the fila- 

 ments. Sometimes they are sessile immediately upon the latter, sometimes they 

 are raised upon very short branches. They are globose, with rather thick walls. 

 Possibly, however, I am mistaken in believing these bodies to be heterocysts, for 

 they may be rather of the nature of spores, as is somewhat indicated by their 

 thick walls, and often apparently dense contents. Their round shape, and the 

 absence of anything else representing heterocysts, has induced me, however, so to 

 consider them. In my Prodromus I placed this plant provisionally amongst the 

 nostocs ; but the radiation of the filaments from within outwards, and especially 

 their being branched, on second thought seem to me to indicate a closer relation 

 with the Rivulariacece. The genus appears to be a sort of connecting link be- 

 tween the two families. 



1 "Flora," 1863. 



