36 



FRESH-WATER ALG^ OF THE UNITED STATES. 



sheath to the filament. These larger threads are apparently produced from the 

 smaller filaments by a process of growth. 



Near the base and in the under portions of the fronds, these filaments are scat- 

 tered in the homogeneous jelly in which they run infinitely diverse courses. In 

 the upper portions of the frond, and at some little distance from the base, the ad- 

 joining cells are very close to one another, and pursue more or less parallel courses, 

 with enough firm jelly between to unite them into a sort of membrane. 



This plant certainly belongs to the NosfocJiacece, and seems a sort of connecting 

 link between the genera Ilormosiplion of Kiitzing and Nostoc. 



The best algologists noAV refuse to recognize the former group as generically 

 distinct; and the characters presented by this plant seem to corroborate that view. 



Adherent to, and often more or less imbedded in, the fronds of the Nostoc, were 

 scattered frustules of several species of diatoms, none of which was I able to iden- 

 tify. In some of the fronds there were numerous unicellular Algae, all of them 

 representatives of a single species belonging to the genus Chroococcus, Niigeli. 

 This genus contains the very lowest known organisms — simple cells without nuclei, 

 multiplying, as far as known, only by cell-division. These cells are found single or 

 associated in small families ; and in certain species these families are united to form 

 a sort of indeterminate gelatinous stratum. In these species the families are com- 

 posed of but very few cells, surrounded by a very large, more or less globular or 

 elliptical mass of transparent, firm jelly. The species is very closely allied to 

 Chroococcus turgidi(s,\av. thermalis, llabenh., from which it difiers in the outer 

 jelly not being lamellated. 



The technical description of this plant will be found in the proper place. 



Fig. 2 a, pi. 2, represents the most mature and largest filament ; Fig. 2 ft, a small 

 filament from the same frond, each magnified 800 diameters. Fig. 2 c, represents 

 portions of the upper surfaces of fronds. 



IV. coinininiituiu, Kxz. 



N. thallo indefinite expanse, gelatinoso, natante, modo viridc, plernmque sordide ferrugineo; 

 trichomatibus flexuosis, plerumque subdense intrieatis; articulis globosis (ante divisionem 

 factam subcylindrieis), subtiliter granulatis, interdum Itete viridibus, plerumque ferrugineis 

 aut luteo-fuscescentibus aut fuscis ; cellulis perdurantibus globosis, articulorum diametro 

 duplo majoribus, interjectis aut termiualibus. 



Diam.—kxiic. ^^Vts"; cell, perdur. ^^Vtt"- 



Sijn—N. comminulum, Kxz. Rabenhokst, Flora Europ. Algarum, Sect. II. p. 119. 



Eab.—ln fossis natante, prope Philadelphia. 



Thallus indefinitely expanded, gelatinous, floating, mostly sordidly ferruginous, sometimes 

 greenish; trichomata flexuous, mostly subdensely intricate; joints globose (before division 

 subcylindrical), minutely granulate, sometimes bright green, sometimes ferruginous, yellow- 

 ish-fuscous, or fuscous ; heterocysts globose, about twice as long as ordinary joints, both 

 interspersed and terminal. 



RemarJcs.— This species is to be found floating on the surface of the ditches 

 below the city in the latter part of August and September, forming a repulsive, 

 ferruginous, slimy scum. The periderm is not very apparent, and indeed the sepa- 



