96 FRESH-WATER ALG^. 



Equally interesting is the life history of Stepha-nosphcEra^ but 

 our space forbids our dwelling on it : it has been calculated that 

 in eight days, under favourable circumstances, more than sixteen 

 million famiUes might be formed from one resting cell. 



Pediastriim has also been included in this class, on account of 

 its general affinities, for the phenomenon of conjugation has not 

 yet been observed in it. When the coenobia multiply a large 

 number of zoogonidia are formed in each mother cell, within 

 which they move about for some time, ultimately coming to rest, 

 congregated in some definite arrangement, ]^and continuing their 

 development unitedly. 



The same uncertainty applies with regard to Hydrodidyon, 

 which forms a beautiful object, Hke a miniature green net, the 

 meshes being very distinct. Dr. Wood, in his " American Fresh 

 Water Algss," says that this genus grows in great abundance in 

 Philadelphia, in the ditches and stagnant brick-ponds in the low 

 grounds below the city, where it forms floating masses several 

 inches thick, and many feet in extent, of a yellowish-green colour. 

 It is in great profusion in June and July, is hardly to be found in 

 the autumn, but reappears early in the spring. 



Mesocarpiis and Zygmena both consist of cylindrical segmented 

 filaments of a green colour, and in both the phenomenon of 

 conjugation may be well observed, as it may be also in the well- 

 known form Spirogyra and its allied genus Rhyiicoiiema. In 

 Characiiim the zoogonidia, after swarming in the mother cell, 

 escape by a lateral rupture, and move rapidly about with the aid 

 of their cilia : no conjugation has been observed. This form is 

 frequently found attached to other filiform algae. 



Hydruriis is apparently propagated only by means of gonidia ; 

 the thallus is elongated, covered with delicate fibres, and of a 

 bright green colour and of gelatinous consistence ; the fronds are 

 often found in large clusters. 



Class III. — Oospores. 



The Thallus may consist of undifferentiated cells, or of a 

 single tubular cell, which often branches freely. In the higher 

 forms the thallus consists of branched and segmented filaments 

 composed of cells of different kinds, and the plant, which is 



