GROUP I. THALLOPHYTA: ALGM : CYANOPHYCE^. 233 



Each spore is formed from a single cell of the body, which sur- 

 rounds itself with a thick firm exospore ; the spore germinates 

 under favourable conditions, the exospore being ruptured (Fig. 

 165 (7). 



It is possible that zoospores are produced in some forms, but 

 the evidence is at present inconclusive. No form of sexual repro- 

 duction has been observed in any member of this sub-class. 



The cells of the Cyanophyceae are nucleated, but the chlorophyll 

 and the phycocyanin appear to be diffused throughout the cyto- 

 plasm, and not to be aggregated in special plastids. 



The Cyanophycese are classified as follows : 

 Order 1. Chroococcaceae. Unicellular : spores formed. 

 Order 2. Nostocaceae. Filamentous ; filaments with heterocysts, without 

 distinction of base and apex, wavy, unbranched ; growth intercalary ; re- 

 production by hormogouia and spores ( Fig. 165). 



Order 3. Oscillariaceae. Filamentous : filaments isolated, motile, without 

 heterocysts, without distinction of base and apex, straight, rigid, un- 

 branched ; growth intercalary ; reproduction by hormogonia ; no spores. 

 Order 4. Rivulariaceae. Filamentous: filaments with heterocysts, and dis- 

 tinction of base and apex, spuriously branched ; growth intercalary ; 

 reproduction by hormogonia and spores. 



OrderS. Scytonemaceae. Filamentous: filaments with heterocysts, with 

 distinction of base and apex in some forms (Sirosiphoneae) but not in others 

 (Scytonemeae), branched spuriously (Scytonemeae) or truly (Sirosiphoneae), 

 growth apical (both ends in Scytonemea?) ; reproduction by hormogonia 

 and spores. 



It is doubtful to what extent these orders really represent distinct forms, for 

 there is evidence that some Cyanophyceas assume the different forms character- 

 istic of these orders at various stages in their development, and under various 

 external conditions, that is, that some at least of the Cyanophyceae are poly- 

 morphic, and this renders their classification uncertain. 



The Cyanophyceee resemble the Schizomycetes, among the Fungi, 

 in many respects; as, for instance, in their general form and struc- 

 ture, in their vegetative multiplication, in their spore-formation, 

 in the absence of sexual reproduction, in the formation of a bulky 

 mucilaginous cell-wall, and in their polymorphism. On these 

 grounds they are frequently placed, along with the Schizomycetes, 

 in a distinct class Schizophyta. But this arrangement does not 

 seem to secure any special advantage. It is more natural to regard 

 the Cyanophyceae and the Schizomycetes as parallel groups, the 

 one belongirig to the Alga3, the other to the Fungi. 



Some of the Cyanophyceee are endophytic, that is, they inhabit the 



