138 PHAEOPHYCEAE 



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 species is readily distinguished from Ectocarpus by the 

 position of the sporangia because these bodies are nearly always 

 intercalary, 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 



SEASON HOST CHROM. NO. SPORANGIUM SWARMER 



Spring 



Early 

 Summer 



Late 

 Summer 



>- cx-^^ 



Ascophyllum or ^ Diploid ^m ^ 0<-^, 



F. vesiculosus 2x n ?^ ' 



Short ^ ^ 1 1 - J 



circuit I ^L R 



" ^ -- " ^ 05---, 



» V li 

 Fucus serratus •> Diploid 2x [j >-Q "^" ^^^^lC-^ 



Seasonal drift -> 



Repetition > 



Fig. 92. Pylaiella littoralis. The life cycle according to Knight. 



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

 tide, high temperatures acting in an inhibitory manner. Meiosis 

 takes place in the unilocular sporangia, and each zoospore when it 

 finally emerges possesses one nucleus, two plastids and flagellae 

 and one eye-spot. After emergence the zoospores usually germinate 

 singly but they have been known to fuse and thus restore the 

 diploid condition. The plurilocular sporangia, which are produced 

 on haploid or diploid plants, are oblong or irregularly cylindrical 

 and also dehisce laterally, each cell producing one zooid which 

 emerges singly. The zooids from these sporangia either fuse or else 

 develop at once, the parthenogenetic zooids arising from diploid 

 sporangia, principally during the summer in England and through- 

 out the winter in Sweden, although isolated cases may occur at any 

 time in the year. The other zooids, which function as gametes or 

 which may occasionally develop parthenogenetically, arise from 



