THALLOPHYTA— ALG^ 59 



oogonium. There is not thus the differentiation of the organs 

 seen in Vaucheria. The antheridia produce a large number of 

 antherozoids, and the oogonia develop several oospheres in each. 

 Fertilisation takes place by the entry of the antherozoids into 

 the oogonia through openings in the walls of the latter, 

 Unlike Vaucheria, this plant shows an alternation of genera- 

 tions, the oospore not producing at once a new Sphaeroplea 

 plant, but giving rise to free-swimming zoospores, which form 

 the new filament when they come to rest and germinate. The 

 oospore thus represents the sporophyte. 



Asexual reproduction is not common ; when it takes place it 

 is brought about by the formation of zoospores. These are gene- 

 rally motile and have a varying number of cilia. In Vaucheria 

 {fig. 805), a single zoospore or zooccenocyte is formed at the end 

 of a filament, a portion of which becomes segmented off by a 

 wall. The protoplasm of this portion midergoes rejuvenes- 

 cence, and becomes ciliated all over, a pair of cilia being placed 

 over each nucleus of the new coenocyte. It escapes by rupture 

 of the apex of the filament. In some forms the asexual gonidia 

 are non-motile. 



CoNFERVOiDE^. — The forms included in this group are either 

 unicellular, filamentous, or membranous ; the filaments are 

 sometimes branched, sometimes not, the membranous ones 

 sometimes form flat plates, sometimes hollow or tubular expan- 

 sions. The growth of the filaments is generally intercalar}'. 

 They are always composed of cells and not of coenocytes. 



The processes of reproduction are both sexual and asexual. 

 The origination of sexuality' can be traced in some members of the 

 group. In TJlothrix, which is a filamentous multicellular form, 

 the contents of certain cells of the filament give rise to a large 

 number of free-swimming ciliated cells, which indicate in their 

 behaviour the beginning of sexual differentiation. They often 

 conjugate in pairs {fig. 80G), forming zygospores and thus indi- 

 catmg that they are essentially gametes. If they do not succeed 

 in conjugating, the^^^ stiU can germinate ; a fact which shows 

 that they are much akin to ordinary zoogonidia. 



A further evolution of sexuality is found in the Zygnemicp, 

 to which Sjnrogyr-a belongs. Any cell of the filaments of the 

 plant can become a gametangium. In the process of conjugation 

 two filaments come to lie parallel with each other, and from the 

 middle of each cell a lateral process grows out towards the other 

 filament {fig. 807, a a). The opposite processes join and their 

 walls fuse, forming a tube which stretches across between the 



