INTRODUCTION 



21 



The occurrence of sexual reproduction in Nature often marks the 

 phase of maximum abundance when the cHmax of vegetative activity 

 has just been passed. It can also be brought on in culture by an 

 abundance or deficiency of food material or by intense insolation. 

 Interspecific hybrids have been recorded from Spirogyra, Ulothrix, 

 Stigeoclonium, Draparnaldia and Chlamydomonas (fig. 14). Another 

 striking fact is that characters which may develop in some species 

 under the influence of the external environment are normally found 

 "fixed" in others. This not only indicates the plasticity of many 

 members in the class, but the phenomenon might also be of im- 

 portance in considerations of phylogenetic relationships. 



Vegetative evolution would appear to have taken place along 

 several lines and may be represented schematically thus : 



Motile colourless 



unicell 



(Phacotus) 



Motile holophvtic unicell 



Palmelloid 



colony 

 (Palmella) 



Small motile 



colony 

 {Pandorina) 



Non-motile unicell 



with motile spores 



(Chlorococcuni) 



>. 

 Large motile 



colony 



( VoJvox) 



Net-like 



colony 

 {Hydrodictyon) 



Attached 



unicell 



(Characium) 



Specialized simple 

 filament (Oedogoniales) 



Foliose parenchymatous 

 thallus (Ulvales) 



Simple filament 

 (Ulothrix) 



Dendroid 



colony 



{Prasinocladus) 



Small siphonaceous 

 thallus {Protosiphon) 



Branched 



filament 



(uninucleate) 



Branched filament 



(multinucleate) 

 (Siphonocladiales) 



Heterotrichous filament 



(aerial and basal portion) 



(Stigeoclonium) 



n 



C 



r Basal cushion 

 Reduced types -j only 



I (Coleochaete) 



Aerial filament 



only 

 (Draparnaldia) 



Siphonaceous 

 thallus (Siphonales) 



Basal disk 



only 



(Protoderma) 



