FRESH-WATER ALG^ OF THE UNITED STATES. 



185 



large trichoma is formed by a continuation of growth, and, perhaps, by a repetition 

 of the division. I have never been able to discover that any reproductive process 

 whatever takes place in this compound filament, and am very confident it never 

 produces zoospores. It is very possible, however, that it may in some way give 

 origin to resting spores, although, as above stated, no indication of this has ever 

 come under my notice. The zoospores are formed in the young fronds as follows : 

 The endochrome in the cell concerned gradually separates in the ordinary manner 

 into several distinct masses, which soon assume a more or less irregularly globular 

 or pyriform shape. Whether the number of these masses is fixed for the single 

 cell or not I am unable to state. These changes occur almost simultaneously in a 

 number of consecutive cells, commencing with the most distal and rapidly spread- 

 ing towards the base of the filament. When they are pretty well advanced, the 

 walls of the cells undergo some alteration, probably a gummy degeneration, whereby 

 they become soluble in the water. As the division of the endochrome occurs first 

 in the most distal cells of the filament, so does also this change in the cellulose coat. 

 When the endochrome masses are well shapen and distinct, they begin to exhibit 

 motion, becoming uneasy, restless, changing their position, rolling on themselves, 

 and pushing against one another. At the same time solution of the cell walls com- 

 mences, the partitions between the cells disappearing, and the outer walls spread- 

 ing. These changes go rapidly forward, and in a little while the zoospores stream 

 out from the fading end of the frond, jostling and crowding as though eager to 

 enter upon their new life. 



Fig. 1 a, pi. 1 7, represents the basal portion of an old filament which has failed 

 to form zoospores, magnified 125 diameters. Fig 1 b was drawn from a young 

 filament during the process of forming zoospores ; owing to their rapid motion, the 

 cilia of the latter could not be seen. This figure is enlarged 250 diameters. Fig. 

 1 c represents a zoospore which has just become quiescent, and still retains its 

 cilia, although they have lost their motile power. Fig. 1 d,e, c, represent the very 

 young plant in different stages of growth. They are all magnified 450 diameters. 



S. Leibleinii, Ktz. ? 



S. laete viridis vel saturate nigro-viridis. 



Biani.—Ma.x. y^s". =.08". 



Syn. — S. Leibleinii, Ktjtzinq. Rabenhoest, Flora Europ. Algarnm, Sect. III. p. 311. 



Hab. — In fossis, prope Philadelphia. 



Bright green to deep blackish-green ; largest diameter of the frond x^j". 



Remarks. — Owing to the profusion of zoospores produced by a single filament 

 at one time, it is very usual to find large numbers of the younger plants attached 

 so closely to some central body as to form dense masses of a beautiful green color. 

 The support of these small masses is often entirely concealed, and I have frequently 

 seen them moving freely about the jar, without any apparent cause, until the mystery 

 was solved by finding that some unfortunate snail carried the forest on his back. 



The oldest filaments are perfectly opaque, showing, under the microscope, by 

 transmitted light, no trace of their structure, 



24 August, 1872. 



