158 PHAEOPHYCEAE 



apex through a series of transverse and longitudinal divisions, until 

 finally there is an external layer of rectangular cells arranged in a 

 polysiphonous manner. Unilocular and plurilocular sporangia are 

 formed on short pedicels, usually on separate plants. 



Clint (1927) has studied in some detail the life cycle of S. bipinfiata, 

 which grows epiphytically on Halidrys (cf. p. 203) in the Irish Sea, 

 whilst farther south it frequents Cystoseira (cf. p. 205). Although 

 primarily an epiphyte it is probable that the species is parasitic to 

 a certain extent. Meiosis occurs in the unilocular sporangia, and 

 after the zooids have been ejected all together in a gelatinous mass 

 they fuse in clumps of t^vo to five, the cytology of the clumps being 

 unknown. Isolated spores may germinate, but under these circum- 

 stances the sporeUng soon dies. The plurilocular sporangia are 

 sometimes stalked, and so it is suggested that morphologically they 

 may be equivalent to branches. The zoospores from these sporangia 

 are smaller and only contain two plastids as compared with those 

 from the unilocular sporangia which contain four. Yet another 

 distinction is that they emerge singly and do not fuse, but germinate 

 immediately on settling. 



Reproduction in this species occurs in early summer and late 

 autumn, and whilst a certain amount is now known about its life 

 history it is still a mystery as to how or in what state it survives the 

 winter. In the north Halidrys breaks off in the winter and no trace 

 of any Sphacelaria plants can be found on the stumps. It is now 

 known that the unilocular sporangia are the asexual organs and that 

 these plurilocular sporangia merely reproduce the diploid generation. 

 The morphologically similar gametophyte generation has since been 

 found and it gives rise to isogametes from plurilocular sporangia. 

 There is thus a regular alternation of generations which agrees with 

 the facts for other members of the family. Vegetative reproduction 

 also takes place in this genus by means of modified branches or 

 propagules which are usually pedicellate and triradiate, the actual 

 shape varv'ing for the different species. The tropical species, S. tribu- 

 loides, is said to form the common food of manv Hawaian fishes. 



Cladostephaceae : Cladostephus {dado, shoot; stephus, a crown). 

 Fig. 109. 



The plants, which are bushy in appearance, arise from well- 

 developed holdfasts and are characterized by the ecorticate branches 



