42 FRESII-WATER ALG^ OF TUE UNITED STATES. 



Syn. — Sphaerozyga, (Auctores, partim.) 



Doiichospermum, Thwaite's MSS. Mr. J. Ralfs on the Nostochinem, Ann. Mag. 

 Nat. Hist. 1850, p. 335. 



RemarTcs. — This genus differs from Sphcerozi/ga in that the spores have no rela- 

 tion, in regard to position, with the heterocysts. Professor Rabenhorst, in his 

 Flora, docs not acknowledge it ; but it is very evident that he has neither seen 

 the original paper of Mr. Ilalfs, nor the species upon which the genus was founded, 

 for he mentions none of the latter, either as good species or synonyms, and the 

 memoir itself is not included in his bibliographical list. The generic characters 

 given by myself are essentially those of the original description, with the excep- 

 tion that the filaments in the latter are said to be aggregated into a stratum, which 

 is not true of the American forms herein described. 



D. siibri^idiini, Wood. 



S. nutans; trichomatibus singulis, rectis ant subrectis, minimis, dilute viridibus; articulis 

 cylindraceis aut subglobosis, distinctis ; sporis cyiindraceis, in medio gradatim nonnihil 

 constrictis, singulis aut duplicis, sine cellulis perdurantibus inter se ; cellulis perdnrantibas 

 breve cylindraceis, singulis, distinctis. 



Syn. — Sphxroziga subrigidum, Wood, Prodromns, Proc. Amer. Philos. Soc, 18C9, p. 123. 



X)tam._CelI. veg. trans. ^^Vir" = -00016"; spor. transv. :jgVty"—iAir" = •00023" — .00022"; 

 long. -r5'5!j" = -00066"; cell. perd. transv. ^^Vis" = 00022". 



Sab. — In stagnis props Philadelphia. 



S. Floating; filaments single, straight or straightish, very small, light green; articles cylin- 

 drical or subglobose, distinct; spores single or double, in the middle gradually a little con- 

 stricted, not having a heterocyst between them ; heterocysts shortly cylindrical, single, 

 distinct. 



Remarks. — I have found this species growing in the scum floating upon the 

 ditches below the city. The filaments are always, as I have seen them, scattered. 

 They seem always to be nearly straiglit, or entirely so, and indeed preserve their 

 straightness so constantly as to suggest the name given the species. The spores are 

 very distinct, and all that I have seen were greenish, cylindrical, and constricted in 

 the middle, so that their sides are concave. Their position does not seem to be 

 uniform, any further than that they are amongst the ordinary cells. The heterocysts 

 are large, almost equalling the spores in diameter ; I have never detected hairs on 

 them. This species appears to be most nearly allied to D. Thwaiiesii of Ralfs, from 

 which it differs in not forming a stratum, and in the great proportionate diameter 

 of the heterocysts. I have never seen any measurements of D. ThioaitesiL 



Fig. 2, pi. 3, is a filament, magnified 975 diameters. 



D. polyspernia, (Ktz.) 



S. trichomatibus plerumque subsolitariis, sed intcrdum consoeiatis et intricati-s dilute ca^ruleo- 

 viridibus, subrectis aut varie curvatis et flexuosis; articulis aut subsphsericis aut breve cylin- 

 dricis; cellnlis perdurantibus globosis aut latissime ellipticis, articulorura diametro paulo 

 vel duplo majoribus; sporis plus minus elongatis, cylindraceis— in setate immatura, sparse 

 granu alls, dilute ca^ruleo-viridibus, et cum membrana baud distincta,— in eetate matura dense 

 granulatis et cum membrana subcrassa. 



