GROUP I. THALLOPHYTA : ALG^E : CYANOPHYCE&. 231 



zygospore, on germination, gives rise to a plant, more or less 

 resembling the gametophyte, which bears only asexual reproduc- 

 tive organs (zoosporangia) and is therefore the sporophyte. 



In the Rhodophyceae, there is an alternation of generations, the 

 plant being the gametophyte (either actual or potential), the 

 fructification (cystocarp) developed from the fertilised female 

 organ (procarp) being the sporophyte. The spores of the sporo- 

 phyte (carpospores) give rise on germination to gametophytes. 



In discussing the alternation of generations of the Algse, it 

 must be borne in mind that the gametophyte can, in many cases, 

 reproduce itself by means of gonidia, and that it may not always 

 bear sexual organs : that it may be, in fact, a potential gameto- 

 phyte. Instances of this occur in Coleochaete, the Rhodophyceae, 

 and others. 



Other forms of polymorphism are of frequent occurrence, as in 

 the Cyanophyceae, Confervoideae, Batrachospermum, Lemanea, 

 Characeae (see p. 14). Details of these are given in the special 

 descriptions of the various families, in which also a more detailed 

 account of the alternation of generations will be found. 



Sub-Class I. CYANOPIIYCEJ; (also called Phycochromaceae), or 

 blue-green Algae. The body consists of a single, more or less nearly 

 spherical cell, as in most of the Chroococcaceae (e.g. Gloeocapsa, 

 Fig. 163) ; or it is a multicellular layer one cell thick (e.g. Meris- 

 mopedia) ; or it is filamentous, consisting of a row of cells (e.g. 

 Oscillaria, Nostoc, Rivularia, Scytonema). When the body is 

 filamentous, it sometimes presents a distinction of base and apex 

 (e.g. Rivularia) ; and it is frequently branched, the branching being 

 either spurious (e.g. Rivularia, Scytonemeas) or true (i.e. formation 

 of lateral growing-points, e.g. Sirosiphoneae). In most cases growth 

 and cell-division go on in all the cells of the body, but in the 

 ScytonemaceaB only at the apex. The plant is usually free, but it 

 grows attached in some species of RivulariaceaB and Scytonemaceae. 

 A characteristic feature of the sub- class is the more or less bulky 

 mucilaginous cell-wall which invests the cells and filaments. The 

 filaments of the Oscillariaceae exhibit a gliding, oscillating move- 

 ment, but the mechanism of it is not fully understood. 



Reproduction is mainly effected in a purely vegetative manner. 

 In the unicellular forms (Fig. 163) each cell-division necessarily 

 leads to the formation of new individuals. In the flattened forms 

 (e.g. Merismopedia), when the body reaches a certain limit of size, 



