ECTOCARPALES 



137 



carpus, and for many years the species was included in that genus. 

 The branching is opposite or akernate, but the branches do not 

 end in a mucilage hair as they do in Ectocarpus. Attachment to the 

 host plants or to the substrate is by means of 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 confined principally 



Fig. 91. Pylaiella littoralis. Portion of plant with plurilocular and unilocular 

 sporangia ( x 200). (Original.) 



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 Ascophyllim 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 frequently 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 third, which is a 

 vernal form that 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 numerous, and 



