102 



THE ALGAE 



swarmers whose fate is not known although they are probably 

 gametes. 



Fig. 57 Halimeda. A, plant of H. simulans ( x 33). B, H. discoidea, 

 longitudinal section showing structure ( x 20). C. central filament: 

 two fuse and subsequently divide into three ( x 20). D, cuticle of 

 H. opuntia ( x 132-5). E, H. scabra, termination of filaments ( x 100). 

 F, fruiting plant. G, sporangia. (A, D, E, after Taylor ^ B, C, after 

 Howe; F, G, after Oltmanns.) 



* Caulerpaceae: Caulerpa {caul, stem; erpa, creep). Fig. 58 



Most of the species frequent the quiet shallow waters of the 

 tropics where they are often rooted in sand or mud, but two have 

 migrated to become denizens of the Mediterranean. The prostrate 

 rhizome is attached by means of colourless rhizoids and gives rise 

 to numerous, erect, upright assimilatory shoots with apical growth, 

 the form and arrangement of which may vary very considerably 

 (Fig. 58). Radial branching is regarded as primitive, whilst the more 

 evolved forms of quieter waters possess a bilateral branching 

 system. The genus has been divided by Borgesen into three groups : 

 (a) The species of this group, which grow where there is much 

 mud, possess rhizomes that are vertical or oblique, thus 

 enabling them to reach the surface even when covered suc- 

 cessively by mud (e.g. C. verticillata). 

 (h) The rhizome in these species first branches at some distance 

 from its point of origin and it possesses a pointed apex which 

 aids in boring through sand or mud (e.g. C. cupressoides). 

 (c) The rhizome is richly branched immediately from its point 

 of origin and the various species are principally to be found 

 attached to rocks and coral reefs (e.g. C racemosa). 



