222 FRESH- WATER ALG.E OF THE UNITED STATES. 



Algae growing in streams and rivers. Thallus developing from a confervoid prothalloid filament, 

 setaceous, almost simple or fasciculately branched, hollow, nodose, composed of internal and corti- 

 cal strata of cells. Nodules generally provided with a corona or papilla. Polyspores numerous, 

 fasciculately aggregated in branched moniliform series, germinating without fecundation. 



Genus LEMANEA, BORY. 

 Genus unicum. 



The only genus. 



Remarks. The plants belonging to the genus Lemanea are quite peculiar in 

 aspect and habit. They grow exclusively in fresh water, especially frequenting 

 streams whose current is rapid, and whose waters are chilled by the mountain air. 

 Their frail, tubular, scarcely-branched fronds offer but little resistance to the 

 water, whilst their lower end is swollen into a sort of discoid root, which adheres 

 firmly to the stones. The frond is mostly blackish or brownish, and is formed of 

 two distinct portions or layers, of which the outer or cortical is composed of small 

 closely cohering, colored cells ; the inner of much larger cells, which have thick 

 colorless walls, and are placed so as to leave more or less numerous interspaces. 

 In the immature frond there is also a longitudinal central column, besides some 

 slender many-jointed filaments, passing obliquely through the cavity, but as final 

 development takes place these seem to disappear. The mature frond is alternately 

 contracted and expanded throughout its length. In the narrow portions the inner 

 tissue often blocks up the tube entirely, whilst the dilated parts are loosely filled 

 with the spores, which are produced within the frond. The spores themselves are 

 oval, thickish-walled cells, whose endochrome changes from greenish to a very 

 decided yellow during the process of maturing. They are joined together to form 

 rows or series, which are not simple, but are very much branched, so that from a 

 central basal row arises a complex bush-like mass (pi. 20, fig. 4). These spore- 

 clusters are always distinct, a number of them existing in each sporangial node of 

 the frond. 



Dr. B. Wartmann described, nearly twenty years ago, very fully the way in 

 which the spores germinate and develop into the frond. The first step, according 

 to this authority, consists in the elongation of the spore and the projection of one 

 end, which is soon cut off by the formation of a transverse partition, and consti- 

 tutes a new cell. This multiplying in no strikingly peculiar way soon develops 

 into a branched confervoid filament. A large number of these filaments are gene- 

 rally produced in one place at one time and form a very apparent greenish layer. 

 Finally certain cells in branches of these filaments swell up and become very much 

 broader than their fellows, undergoing, at the same time, division so rapidly that 

 they become very short. By and by they divide also in the direction of their 

 breadth, so that instead of a simple series of cells there arises a compound mass. 

 This is the beginning of the new frond. At first it is dependent upon the parent 

 filament, but soon acquires a root-like process at the base and develops rapidly 

 into the complex cartilaginous plant. 



