GREEN ALGAE 



365 



colonial forms, the remarkable organism, the Water-net, Hydrodictyon 



reticulatuyn, representing the highest state of organisation in the Order. 



The very common unicellular green alga, Pleurococcus Naegelii, Chod. 

 (Protococcus viridis), Fig. 269, found as <. green incrustation on the windward 

 side of tree-trunks, walls, etc., was formerly placed in this group. It is now 

 considered to be a very reduced member of a considerably more advanced 

 group, the Chaetophoraceae. Multiplication is by cell-division, Fig. 269, no 

 motile stage and no resting stage being known to occur. 



Ulotrichales and Oedogoniales. 

 These Algae show some advance in development of their plant-body, 

 owing to repeated cell-division, the products of which remain associated 

 together to form simple fila- 

 ments, as in Ulothrix or Oedo- 

 gonium ; or the filaments 

 may be branched, as in 

 Bulbochaete ; or flattened 

 expansions may be formed, 

 as in Ulva or Enter omorpha. 

 The plants inhabit salt or 

 fresh water, or may even 

 grow in moist air, as Hormi- 

 dium does. Ulothrix, which 

 may be taken as a first ex- 

 ample, is commonly found 

 attached to stones washed 

 by a quickly running stream: 

 but some of its species are 

 marine. The unbranched 

 filament consists of a series 

 of discoid cells, each with a 

 zonal chloroplast, and it is 

 attached by a basal rhizoid 

 (Fig. 270, A). Its propaga- 

 tion though varied is rudi- 

 mentary like its vegetative 

 structure. Motile zoospores 

 may be produced either 

 singly from a cell, or by 

 division of its contents ; 

 they escape through an 

 opening of the cell-wall into 



Fig. 2?o. 



Ulothrix zonata. A, young filament with rhizoid r 

 ( x 300). B, portion of filament with zoospores escaping 

 from a zoosporangium. C, single zoospores. D, forma- 

 tion and escape of gametes from a gametangium. 

 £= gametes. F = conjugation. G= zygote. H= zygote. 

 / = zygote after period of rest. K = zygote after division 

 into zoospores. (After Dodel Port.) (B-K x 482.) (From 

 Strasburger.) 



