1^6 THE ALGAE 



branched threads which gives way to a looser peripheral zone 

 of assimilatory threads (paraphyses of some writers). Outside 

 this is an outermost zone of free, long, assimilatory hairs with 



plastids. 



The various species are epiphytic on other algae, Elachistafucicola 

 being especially abundant on species of Fucus. Unilocular spor- 

 angia arise from the base of the assimilators, the distal portions 

 being modified to form plurilocular sporangia. The zooids from 

 the unilocular sporangia germinate in late autumn to give a 

 branched, thread-like, microscopic gametophyte which persists 

 throughout the winter. In late winter and spring plurilocular 

 sporangia develop on the minute gametophytes, and when the 

 zooids have been Uberated they fuse and the zygote germinates into 

 a new macroscopic Elachista plant. 



Exceptions to the normal type of Ufe cycle are found in E. fuci- 

 cola and E. stellaris. Only unilocular sporangia are known and the 

 zooids develop into protonemata that give rise to new sporophytes 

 as lateral outgrowths. In these cases meiosis cannot take place. The 

 species are epiphytic on other algae, the genus being represented 

 in both hemispheres. 



Myrionemataceae : Myrionema {myrioy numerous ; nemay thread). 

 Fig. 86 

 This represents one of the highly reduced species, but the funda- 

 mental basic cable construction of prostrate thallus and erect 

 threads can be found, though the erect threads do not branch. The 

 genus is of wide distribution, the commonest species being M. 

 strangulans on species of Ulva and Enter omorpha. The various 

 species form thin expansions or minute flattened cushions or discs 

 that are very variable in shape and from which numerous, closely 

 packed, erect filaments and hairs arise. The basal monostromatic 

 portion of the thallus has a marginal growing region and is com- 

 posed of crowded radiating filaments that may, on rare occasions, 

 penetrate the host plant. The unilocular sporangia, which are not 

 borne on the same plants with plurilocular sporangia, give rise to 

 haploid zooids, and these develop into a thread-like gametophytic 

 plant bearing long filaments, the possession of this type of gameto- 

 phyte indicating that the genus is perhaps more closely aUied to the 

 Mesogloiaceae than to the Ectocarpaceae. Although M. strangulans 

 possesses a Ufe cycle typical of the Chordariales, it has been sug- 



