3 



which occurs in the same area but on the coasts of the 

 Irish Sea also. B. hypnoides is recorded from the Isle of 

 Man and Puffin Island, Anglesea, but requires confirma- 

 tion ; B.plumosa is fairly common in rock pools in the 

 L.M.B.C. area. 



II. DERBESIACEiE. 



This order also includes only one genus, Derbesia, with 

 about 8 species, according to Wille (Engler and Prantl, 

 Pfianzen-familien). The thallus is simple, irregularly or 

 dichotomously branched, and produces splierical swarm- 

 spores in lateral zoosporangia. Each zoospore is provided 

 with a circlet of cilia, surrounding the bluntly pointed 

 hyaline apex. 



Derbesia tenuissima, Cm., is recorded from the English 

 Channel, but it has not as yet been discovered in the 

 L.M.B.C. area. 



III. Vaucheriace^. 



This order also is represented by only one genus, 

 Vaucheria, and embraces about 25 species, which are 

 inhabitants of fresh or l)rackish water. Whilst vegetatively 

 not unlike Derbesia, Vaudieria differs markedly in its 

 methods of reproduction from the rest of the Siphoneae. 

 In addition to considerable powers of vegetative multipli- 

 cation, solitary zoospores (zoocoenocytes) are developed in 

 sporangia formed by the abstriction of the terminal por- 

 tions of vegetative branches. Sexual organs in the form 

 of ovoid oogonia and filamentous antheridia also occur as 

 lateral branches of the filamentous thallus. FertiHsation 

 of the solitary ovum takes place within the oogoniun:, the 

 oosperm developing into a new Vaucheria filament after a 

 period of hibernation. 



Five species, with several varieties, are found on British 



