222 THE ALGAE 



Nemalionales reproduction by means of monospores is quite 

 common though the homologies of these bodies are somewhat un- 

 certain. In some of the Euflorideae (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 triploid generation (cf. p. 256). Ex- 

 perimental cultures made on oyster shells have demonstrated that 

 there are good grounds for beHeving that of the four spores in a 

 tetrad, two will give rise to female plants and two to male plants. 

 Observations have been published showing that monospores, car- 

 pospores and tetraspores of some Rhodophyceae appear capable of a 

 small degree of motion, the spores of the Bangiaceae being the most 

 active among those investigated. The mechanism of this move- 

 ment is not understood, and it is doubtful whether it is sufficient 

 to give it any significance in the reproductive 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 Peyssonnelliopsis^ are probably entirely 

 parasitic. In European waters Choreonema, Schmitziella, Choreo- 

 colaxy Harveyella and Holmsella are all to be regarded as partial or 

 total parasites, and to this list Polysiphoniafastigiata should perhaps 

 be added, since it is alv/ays found on one particular host, Asco- 

 phyllum.^ 



The division is principally marine, but there are a few fresh 

 water genera, comprising in all about 200 species. The most im- 

 portant fresh-water genus is Batrachospermum with many species 

 in Australasia, and others are Lemanea and Hildenhrandtia. They 

 nearly all frequent fast-flowing streams where there is an abundance 

 of aeration. 



As in the Chlorophyceae there is one section, the Corallinaceae, 

 that is characterized by lime encrustation. These algae have played 

 a great part during past geological ages in the building up of rocks 

 and coral reefs (cf. p. 238), a process which can still be seen going 

 on in tropical seas today. 



^ A number of very interesting parasites have been recorded from S. Africa. 



