EUPHYCOPHYTA 



133 



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rhizoidal filaments, and near the base the main filaments of the 

 erect thallus are frequently coalesced into a rope-like structure as 

 a result of wave action. In some 

 places the plants appear to be con- 

 fined principally to certain host 

 plants whilst in other areas there 

 may be no special hosts. In the 

 Isle of Man, Knight (1923) has 

 shown that in the spring the plants 

 occur on Ascophyllum nodosum^ 

 in early summer they are to be 

 found on Fucus vesiculosus and in 

 late summer on F. serratus, yet 

 in north Norfolk the species fre- 

 quently grows on the stable mud 

 banks of salt marsh creeks or else 

 on F. vesiculosus. On the Swedish 

 coast three forms have been 

 noted, two of which are found on 



Ascophyllum nodosum, whilst the Fig^2 Pylaiellalittoralis.Fomonsof 

 third, which is a vernal form that plantwithplurilocular and unilocular 



dies off at the end of June, occurs ^^^^^"^^^ ^ ^ "°°^- ^"^''^^'^'^ 

 attached to stones. Of the two forms observed on Ascophyllum it is 

 found that those directly attached to the host are the more numer- 

 ous, and although they persist for the whole year, they are most 

 fertile in winter when they produce unilocular sporangia. The other 

 plants are really epizoic because they grow on the colonies of 

 Sertularia (a hydroid) that are to be found on the Ascophyllum. 

 These plants, which only bear plurilocular sporangia, are most 

 vigorous during spring and early summer and are dead by the end 

 of July. 



This genus is readily distinguished from Ectocarpus by the posi- 

 tion of the sporangia because these bodies are nearly always inter- 

 calary, very rarely terminal, and when this latter is the case it is 

 frequently due to the loss of the terminal vegetative portion. The 

 unilocular sporangia are cask-shaped and open laterally, dehiscence 

 of the sporangium being brought about by the swelling up of the 

 middle layer of the wall, but this process is dependent on the tem- 

 perature of the water when the plant is flooded by the incoming 

 tide, high temperatures acting in an inhibitory manner. Meiosis 



