56 THE ALGAE 



In sexual reproduction, which occurs in plants other than those 

 producing zoospores, fusion normally takes place between iso- 

 gametes from separate plants (+ and -). Thirty-two or 64 

 gametes are produced per cell and in three species anisogamy is 

 known to occur. In some species the gametes may fuse into clumps. 

 In U. lohata two types of *clump' reaction have been observed. In 

 one type 10-20 gametes fuse together whereas in the other more 

 than 100 are involved. At present there is no explanation of this 

 difference. Whilst the gametes are positively photo tactic before 

 fusion, the zygote is negatively phototactic, and this change in 

 behaviour causes it to descend on to a suitable substrate. It has 

 been shown that in certain cases there may be relative sexuahty 

 among gametes from different plants, the sex of the older and 

 weaker gametes becoming changed (see p. 19). Meiosis takes place 

 at zoospore formation and, in all but one of the species investi- 

 gated, there is a regular alternation of morphologically similar 

 diploid and haploid generations (see Fig. 26). In U. lima, only the 

 diploid generation is known and meiosis does not occur at zoospore 

 formation. In U. lohata gamete Hberation occurs with a fourteen- 

 day periodicity, the periods of liberation being associated with 

 spring tidal cycles (see p. 148). 



The plants occur in saline or fresh water and become particularly 

 abundant when the waters are polluted by organic matter or 

 sewage. 



In the aUied sub-antarctic genus Gemina the plants resemble 

 either Ulva or Enter omorpha. The cells are normally in pairs and 

 apparentiy only some cells are capable of reproduction whereas in 

 Ulva all cells of the thallus can give rise to swarmers (Fig. 32). 

 The same is true also of the genus Lohata, the reproductive cells 

 being restricted to the thickened central portion of the thallus. 



Ulvaceae: Enter omorpha {enter o, entrail; morpha, form). Fig. 29 



The plants of this genus also commence Hfe as uniseriate fila- 

 ments which soon become multiseriate and tubular. Like Ulva, 

 many of the species are attached by means of rhizoids and in E. 

 nana (sometimes placed in a separate genus Blidingia), there is a 

 basal prostrate portion, but there are also a number of forms, oc- 

 curring especially on salt marshes (cf. p. 354), which are unattached 

 for the whole or part of their hfe. Growth in length of the thallus 

 is primarily intercalary though in the early stages it is also through 



