FRESH-WATER ALG^ OF THE UNITED STATES. 23 



but, when they are, the joints are shorter than broad. The endochrome is mostly 

 very granulate ; sometimes, however, it is much more homogeneous. The sheaths 

 in the old filaments are rather thick, and frequently partially empty ; the exterior 

 of such sheaths has often a rough, ragged look. The larger cells are very few in 

 number. They are elongated cylinders with concave ends. I have found this plant 

 in the Schuylkill River, just above Fairmount dam, in a younger state, and appa- 

 rently without heterocysts. The threads near their ends had their endochrome 

 distinctly articulate, like an oscillatoria, but elsewhere the protoplasm was continu- 

 ous. It often contains numerous large granules resembling minute starch grains, 

 which however fail to exhibit the reaction with iodine. 



Fig. 7, a, pi. 1, represents a portion of the filament slightly magnified; fig. 7, h, a 

 heterocyst from the same specimen more magnified ; fig. 7, c and d, are drawings from 

 another specimen from the same locality, each magnified 800 diameters; fig. 8, 

 pi. 1, represents the form alluded to in the text as having been found in the 

 Schuylkill lliver just above the dam. 



Family NOSTOCHACEiE. 



Trichomata simplicia, e cellulis distinctis composita, interdum vaginata, articulata, in gelatina 

 immersa, cellulis perdurantibus, et interdum sporis porro instructa. 



Filaments simple, composed of distinct cells, sometimes vaginate, imbedded in jelly ; furnished 

 with heterocysts and sometimes with spores also. 



Bemarks. — The nostochacese are plants of simple construction, consisting of a 

 more or less firm jelly in which are imbedded serpentine filaments, composed of 

 numerous cells. These cells are mostly more or less globose, especially in the true 

 nostocs, so that the filament has a moniliform aspect. They have not distinct walls, 

 or at least any that can be distinctly seen by ordinary powers of the microscope, 

 and are sometimes closely connected, sometimes rather widely separated. No 

 nuclei are usually discernible ; I have, however, seen in some instances central 

 spots, which were possibly of that nature. The filaments themselves are of various 

 length, almost always tortuous, sometimes widely separated, sometimes closely in- 

 terwoven. The gelatinous portions of the fronds are of various consistence — some- 

 times semifluid, sometimes very firm, almost cartilaginous. 



The order is divisible into two families — the Nostocs proper and the Spermosireoe. 



In the former, the outer portion of the frond is condensed and firm, forming a 

 sort of outer coat or epidermis, which is sometimes quite distinct, but in other 

 instances can scarcely be said to exist. 



In the filaments of a true nostoc are placed at irregular intervals cells, which 

 are mostly larger than the others, and have thick, distinct walls. These cells con- 

 tain very little or no chlorophyllous protoplasm. They are often, but by no means 

 always, provided with numerous exceedingly attenuated, hair-like processes, or 

 quiescent cilia. These bodies were supposed by Kiitzing to have some sexual 

 value, and received from him the name of Spermatia. But, as their functions are 

 entirely unknown, the name of heterocysts, first applied by M. AUman, is prefera- 

 ble. They are the " connecting cells" of Thwaites. No one has as yet demon- 



