GROUP I. THALLOPHYTA : ALG.E. 241 



II. Hcterogamy : 



(a) Oogamy : the female organ is an oogonium ; the sexual 

 cells are spermatozoids and oospheres, the former ciliated and 

 free-swimming, the latter not ciliated but sometimes free-floating; 

 process, fertilisation ; product, an oospore ; (e.g. Volvox, Vaucheria, 

 (Edogonium, Coleochsete, Characese, Fucacese). 



(b) Carpogamy ; the female organ is a procarp in which no 

 female cell is differentiated ; male cell free, not ciliated, a sperma- 

 tium ; process, fertilisation ; product, a fructification termed a 

 cystocarp (Rhodophycese). 



The sexual cells are aplanogametes, planogametes, oospheres, 

 spermatozoids and spennatia ; though they differ widely in various 

 respects, they agree in being nucleated masses of protoplasm 

 destitute of a proper cell-wall. 



The aplanogametes are characterised by the absence of cilia 

 and of any defined form ; they are confined to the Conjugates and 

 Diatomacese. 



The planogametes are somewhat pear-shaped, the anterior more 

 pointed end being destitute of the chromatophores which are pre- 

 sent in the more rounded portion. They have two cilia which 

 are inserted, in the isogamous Chlorophyceee, at the pointed end 

 of the cell ; in the isogamous Phseophycese, laterally at the junction 

 of the anterior colourless portion with the posterior coloured por- 

 tion of the cell. In conjugation, the planogametes first come into 

 contact by their colourless anterior ends. 



The oospheres are spherical cells, usually containing chroma- 

 tophores either throughout their whole substance, or leaving a 

 colourless area on one side, the receptive spot, at which the sper- 

 matozoid enters in the process of fertilisation (e.g. (Edogonium, 

 Vaucheria, Sphaeroplea). 



The spermatozoids may be somewhat pear-shaded, resembling 

 the zoospores of the plant, but smaller (e.g. Coleochsete, (Edogo- 

 nium) ; or they may be more elongated and club-shaped (e.g. 

 Sphseroplea, Volvox) ; or still more elongated and spirally twisted 

 (Characeae). They usually bear two cilia at the pointed end ; but 

 in Vaucheria, Volvox, and the Fucacese, they are inserted laterally ; 

 in (Eiogonium there is a circlet of cilia round the pointed colour- 

 less end. They are faintly coloured, in the Chlorophycese usually 

 yellow. 



The male cells of the Rhodophycese are peculiar on account of 

 the absence of cilia, and are distinguished by the special name 



M.B. R 



