FRESH-WATER ALG^ OF THE UNITED STATES. 173 



I have met with an American plant, which has some of these characters, and at the 

 same time others which have been supposed to belong to the genus Spirogyra. It 

 unites the method of reproduction of Sirogonium and the arrangement of the 

 chlorophyl band of Spirogyra, standing as it were midway between them. It is 

 not midway, however, but much nearer Sirogonium, for the passage from a very 

 loose spiral to a longitudinal flexuous filament is a brief one, and although in some 

 cells of S. retroversum the spiral makes a number of turns ; in other long cells it 

 scarcely gets around once, in other words the chlorophyl band is nearly straight. 

 On the other hand, the reproduction is strictly that of S. strictum, at least in 

 all cases which have come under my notice. There is, therefore, but one of two 

 things to be done, either to unite Sirogonium with Spirogyra, or else to give up 

 the arrangement of the chlorophyl as an essential character of the former genus. 

 The great variance, in the latter respect, in our American species, greatly weakens 

 the value of any such character, and I have, therefore, preferred the latter of the 

 two courses. 



S. retroTersum, Wood. 



S. articulis sterilibus diametro t-lS plo longioribus ; fesciis spiralibus 1, rare 2, latis, granu- 

 latis ; anfractibus 1-9 ; articulis fertiljbus valde tumidis, retroversis ; conjugatione genuflexa et 

 sine tube connexivo ; cytiodermate nonnihil crasso, utroque fine protenso vel replicato ; 

 sporis elliptieis. 



Diam.— Art. steril. ^ii^" =.00146"; spor. lat. 7io/_^i§/=.00133"— .0016" ; long. ^§4/ 

 = 0033". 



Syn. — S. retroversum, Wood, Prodromns, Proc. Am. Phil. Soc. 1869, p. 139 



Hub. — In stagnis, prope Philadelphia. 



Sterile joints 1-15 times longer than broad; chlorophyl band 1, rarely 2, broad, granulate; 

 turns 1-9 ; fertile article very tumid, retroverted ; fertile cells scarcely swollen ; cell wall folded 

 in at the ends ; chlorophyl band single ; turns 6 ; spores oblong or elliptical, spore wall very 

 thick. 



Remarks. — I have found this species growing in stagnant ditches in the Neck 

 below the city. In fruit the cells are almost always very markedly bent backwards, 

 and have a broad pouch-like dilatation in front. The spores are elliptical, and, as 

 I have seen them, greenish and with a thin coat, but m,ay not have been completely 

 matured. 



Fig. 1, pi. 16, represents this species. 



Genus MESOCARPUS, Hassall. 



Cellulse massa chlorophyllosa initio diffusa, postea in fasciam longitudinalem, hand raro flexuosam 

 contraeta; nucleum centralem et granum amylaceum unicum vel duo involvens. Zygospora globosa 

 vel ovata, in tubo connexivo inter cellulas binas plus minus genuflexas formata. 



Chlorophyl mass in the beginning diffused in the cell, afterwards contracted into an often flexuous 

 fascia, and involving a central nucleus and one or more starch granules. Zygospore globose or 

 ovate, formed in the connecting tube between two more or less bent cells. 



M. scalaris, Hassall. 



M. cellulis sterilibus diametro 3-6 plo longioribus, fertilibus valde curvatis ; zygosporis ovalibiis. 



Diam.—Max. yf^^ = .0011". 



