2i6 RHODOPHYCEAE 



agent: this feature has led to the suggestion that in the more 

 advanced red algae the contents of the antheridium are equivalent 

 to a body which formerly did divide. 



The tetraspores are either formed in superficial tetrasporangia or 

 else they are sunk into the thallus, in which case the fertile branch 

 often becomes swollen and irregular in outline, whilst in the 

 genus Plocamium there are special lateral fertile branches or 

 stichidia. Meiosis normally occurs at the formation of the tetra- 

 spores, but when the spores develop on sexual haploid plants, as 

 sometimes happens, there is no meiotic division and the products 

 function as monospores. In Agardhiella tenera apospory is some- 

 times found and again there is no meiosis so that a succession of 

 asexual plants can occur. In the Nemalionales reproduction by 

 means of monospores is quite common though the homologies of 

 these bodies are somewhat uncertain. In some of the Eu-florideae 

 (Plumaria, Spermothamnion) polyspores or paraspores develop on 

 the diploid plants, but it has recently been shown that these are in 

 some cases morphologically equivalent to tetraspores, whilst in 

 others, e.g. Plumaria, they form the reproductive organs of a tri- 

 ploid generation (cf. p. 238). Experimental cultures made on oyster 

 shells have demonstrated that there are good grounds for believing 

 that of the four spores in a tetrad two will give rise to female plants 

 and two to male plants. Observ^ations have been published showing 

 that monospores, carpospores and tetraspores of some Rhodo- 

 phyceae appear capable of a small degree of motion, the spores of 

 the Bangiaceae being the most active among those investigated. The 

 mechanism of this movement is not understood, and it is doubtful 

 whether it is sufficient to give it any significance in the reproduc- 

 tive processes of the plants. 



Whilst there are apparently very few truly parasitic species 

 among the Chlorophyceae and Phaeophyceae, nevertheless in the 

 present group there are some very definite partial or total parasites. 

 Ceramium codicola occurs on a Californian species of Codium and is 

 said to be a partial parasite ; Ricardia Montagnei is probably a total 

 parasite at some stage of its existence, and the members of the two 

 genera, Janczewskia and Peysonielliopsis, are probably entirely 

 parasitic. In European waters Choreonema, Schmitziella, Choreo- 

 colax, Harvey ella and Holmsella are all to be regarded as partial or 

 total parasites, and to this list Polysiphonia fastigiata should per- 



