SEASON HOST CHROM. NO. SPORANGIUM SWARMER 



>- cx-^ 



: CX ] 



Early 

 Summer 



Late 



Summer 



f -rr- 



Ascophyllum or ^ Diploid ^m ^ 0<— 



F. vesiculosus 2x j( 9^ * 



Short ^^ ^ 1 i _; 



circuit y ^ R 



Fucus serratus ->• Diploid 2a; 41- — »-0 ^2^ > 



{ ^ D V S^\ 



-~ — — — ^ — — — ^— ^ ^ — — — — — — — — ^ — _ _ _ __ ^ _ _ _ _ _ m^ 



Seasonal drift ■> 



Repetition > 



Fig. 73 Pylaiella littoralis. The life cycle according to Knight. 



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 dip- 

 loid 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 throughout 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 haploid 

 sporangia and are most abundant in spring and early summer. 

 Fig. 73 is a schema to illustrate the life cycle as foimd in English 

 plants during the course of one year. In the related species P. 

 rupinicola, only the diploid generation is known and the sexual 

 haploid generation has apparently been lost. In P. {Bachelotia) 

 fulvescens the sporangia may be borne in biseriate instead of uni- 

 seriate rows. For this reason the plants are commonly placed in a 

 subgenus. 



EcTOCARPACEAE : Phaeostroma {phaeo, brown; stroma, mattress). 



Fig- 74 



This is cited as an example of a much reduced ectocarpoid form, 



which occurs as an epiphyte or partial parasite upon marine grasses, 



such as Zostera, or else upon other brown algae. The filaments, 



although creeping, are clearly ectocarpalean and in places may ter- 



134 



