272 



EMBEYOGENY IN ALG^. 



These zoospores swim about, and then fix themselves, giving rise to 

 yonng Confervse. This is a first asexual generation. The young 

 Conferva is a sort of prothallium, for it bears certain sexual organs. 

 One kind of organ presents itself in the form of cells covered by a 

 membrane, pierced with a certain number of apertures, and having 

 contents which become converted into spores. These are the arche- 

 gonia (fig. 480 6). A second kind has a membrane also pierced with 

 several apertures, and contains small mobile baculiform (rod-shaped) 

 bodies. These are the antheridia, with their spermatozoids (fig. 480 a). 

 The spermatozoids come out from the cells, and enter the openings in 

 the spore-bearing cells, and thus fertilise the spores. 



Saprolegnieae, including the genera Achlya, Saprolegnia, and Py- 

 thium, are cellular plants which grow on dead and living animals. 

 The name is derived from ffatrgss, putrid, and Xeyvov, a coloured border. 

 The bodies of flies thrown into water often 

 become covered with these minute thread- 

 like organisms. Gold fish in tanks have 

 their gills sometimes covered with Achlya 

 prolifera. They resemble in appearance 

 the mucors or moulds, and some have 

 placed them amongst the Fungi. They 

 seem, however, to be more nearly allied 

 to filamentous Algse, such as Vaucheria. 

 At the end of the filaments a cell is 

 formed, which becomes separated from the 

 rest of the filament by a septum. Zoo- 

 spores (fig. 481 a a) are developed, which 

 escape by the bursting of the cell. The 

 filaments of Saprolegniese also produce 

 lateral branches, at the ends of which are 

 swellings, which are divided from the rest 

 of the tissue. In them sacs called oosporangia are formed (fig. 481 5). 

 These are fertilised by the union of cells containing spermatozoids, in the 

 same way as Vaucheria, and oogones {dih, an egg) are formed. Thus 

 there are two modes of reproduction — one by asexual zoospores (fig. 481 

 a b), and the other by sexual antheridia and oosporangia (fig. 481 c d). 

 In the red sea-weeds, called Rhodospermeae or Ploridese, fecunda- 

 tion is effected by antheridia, containing motionless corpuscles, and a 

 peculiar hair-like body called trichogynium (^g/^, hair, yuv>i, female). 

 At the base of this latter organ there is a cell which, after 

 fertilisation, is transformed into the cystocarp (xvsrig, a bladder 



Fig. 480 a &, Sphasroplea (mnvXi/na. Male filament, a, consisting of cells with vacuoles, 

 and with spermatozoids which are passing out ot the cells by openings in the walls. Female 

 filament, &, formed by cells containing spores, which are being fertilised by the spermato- 

 zoids, which enter the cells by openings in the walls, and come in contact with the cellular 

 spores. 



Fig. 480. 



