166 SEAWEEDS 



zoospores each with four cilia. Vegetative propaga- 

 tion also takes place. 



The Thallus. In Chcetomorpha the thallus remains 

 unbranched, in Ehizoclonium there are very short 

 branches, while in Vrospora there are mere lateral 

 outgrowths from the cells, not cut off by cross-walls ; 

 in Cladophora the thallus, is densely branched, the 

 branches being given off at the upper part of the cell, 

 just below the cross- wall separating it from the one 

 above. The base, which is attached to the substratum 

 by a holdfast, is in some cases, e.g Cladophora 

 rupe-striSj much strengthened by the growing to- 

 gether of a number of rhizoid filaments. The 

 filaments of Gomontia, which inhabits the shells of 

 molluscs, penetrating their substance very densely, 

 are less regularly branched, and the cells become 

 irregularly swollen, sometimes more or less beaded. 

 The whole thallus may be easily isolated by placing 

 a fragment of the green-stained shell in Perenyi's 

 fluid (see p. 30). 



The number of nuclei in each cell varies consider- 

 ably, and the chromatophores also vary in shape, 

 but are mostly perforated and flat, or net-like, with 

 abundant pyrenoids. 



The Reproductive Organs. Gametes are produced 

 by both Cladophora and Urospora. They arise within 

 the vegetative cells in large numbers, and escape by 

 a round hole ; they are bi-ciliated and have a red 

 spot. They are all exactly alike in appearance, and 

 conjugate in pairs. The zygote germinates at once 

 in Cladophora, and after a period of rest in Urospoiw. 

 Zoospores are known in all the genera except Rhizo- 



