30 FRESH-WATER ALG^E OP THE UNITED STATES. 



Sutlierlandli a nostoc has been described by Mr. Dickie, which was collected in the 

 neighborhood of Baffin's Bay, and must be referred to this species, although the 

 description given of it is very imperfect. Again, N. alpinum, KTZ., appears to be in 

 all respects similar to the North American forms. So that this cosmopolitan little" 

 plant seems only to ask for a cold shelter, and it nourishes. The Alps, the Alle- 

 ghanies, the Rocky Mountains, and the cold North are its homes. To those who 

 believe in a single centre for a species, the suggestion that it has spread across 

 the globe, through the arctic regions, and followed our mountain chains southward, 

 will of course present itself. 



As I have seen it, the plant is very abundant where it grows, five, six, twelve, 

 or more of the little fronds adhering to a single pebble. The frond is generally 

 longer than broad, the margin sometimes sinuous but never, as I have seen it, 

 lobate or incised. It appears finally to burst and discharge its inner portion, 

 whilst the outer cortical portion, now a little vesicle containing a globule of air, is 

 set free and floats down the stream. 



I. depressum, WOOD, (sp. nov.) 



N. euormiter suborbicularc, minutum, gregarium et interdum aggrcgatum mnscos immersos 

 adhserens, mangitudine seminis sinapcos vcl parvius, durum, clasticum, subnigris; pcridermate 

 firme, achroo; trichomatibus plerumque laxe intricutis, baud vaginatis; articulis globosis, 

 plerumque modice arete connexis, rare distantibus; cellulis pcrdurantibus globosis, ccteris 

 paulo majoribus. 



Diam. Artie, veget. max: .0002"; cell, perdurant. max .00029. 

 Hob. In rivulis, New Jersey (Prof. Austin). 



Irregularly suborbicular, gregarious and sometimes aggregated, elastic, blackish, about the size 

 of a mustard-seed, or smaller, adhering to immersed mosses ; periderm firm, translucent ; fila- 

 ments not vaginate, mostly loosely interwoven ; joints globose, generally rather closely con- 

 nected, rarely distant ; heterocysts rather larger than the other. 



Remarks. This plant was found by Prof. Austin attached to a brook-moss 

 (DicJielyma), growing in a rapid rivulet in Northern New Jersey. 



The minute fronds sometimes are so thin and spread out as to be almost folia- 

 ceous. The species I take to be most nearly allied to N. lichenoides of Europe, 

 from which it is, however, apparently distinct. In the American plant the fila- 

 ments and heterocysts arc a little larger, and the frequent elliptical cells of the 

 European plant are wanting. The frond also apparently does not grow so large as 

 the European, and is further distinguished by its flat, discoid form. In many of the 

 specimens examined the filaments are very thick, irregular, and contorted, the cells 

 being fused together. In other instances, a filament will be plainly double, and 

 every grade between these conditions is present. This is plainly owing to a process 

 of growth, to the cells enlarging and dividing laterally so as to form new filaments. 



W. sphaericnm, (POIRET,) VAUCH. 



N. globosum, interdum oblongum vel ovale, gregarium, ssepius aggrcgatum, raro tamen conflu- 

 ente, durum, elasticum (in state provccta intusmollc ct subaquosum?), olivaceum, niagnitudinc 

 seminis sinapeos, ad cerasi parvi ; peridermate firmo, pellucido; trichomatibus intricatis, luteo- 

 lis, aut prasinatis aut dilute cseruleis; articulis plerumque subquadratis, interdum transverse 



