20 THE ALGAE 



activity has just been passed. It can also be induced in culture by 

 an abundance or deficiency of food material or by intense insola- 

 tion. Interspecific hybrids have been recorded in Spirogyra, Ulo- 

 thrix, Stigeoclonium, Draparnaldia, and Chlamydomonas (Fig. 2). 

 Another striking fact is that characters v^hich may develop in some 

 species under the influences of the external environment are 

 normally found 'fixed' in others. This not only indicates the plas- 

 ticity of many members in the class, but the phenomenon may also 

 be of importance phylogenetically. 



As a group the Chlorophyceae are very widespread, occurring in 

 all types of habitat, marine, fresh-v^^ater, soil and subaerial. Only 

 about 10 per cent are marine and are mainly represented by the 

 Ulvaceae, Siphonocladales, Dasycladales and Siphonales. Many of 

 the marine species have a definite geographical distribution where- 

 as most of the fresh-water and soil algae are cosmopoHtan. A few 

 genera, e.g. Entocladia {Endoderma\ Chlorochytrium^ Cephaleuros^ 

 Phyllosiphon, contain parasitic species, whilst species of other 

 genera participate in symbiotic associations, e.g. Carteria, Zoo- 

 chlorella, Trehouxia (cf. p. 427). 



* VOLVOCALES 



This order comprises the simplest members of the Chloro- 

 phyceae and is the only order of green algae (Chlorophyceae) with 

 motile vegetative cells. It ranges from simple unicellular forms of a 

 basic type, e.g. Chlamydomonas ^ to regular colonial aggregates of 

 similar cells, e.g. Volvox, whilst in other cases the cells are ag- 

 gregated in an irregtilar manner, e.g. Tetraspora, or in a dendroid 

 form, e.g. Prasinocladus. Throughout the order the motile repro- 

 ductive swarmer is very uniform in character and even when ag- 

 gregated the individual cells commonly retain their power of loco- 

 motion. 



* Chlamydomonadaceae: Chlamydomonas (chlamydo, cloak; 

 monasy single). Fig. 2 

 The chlamydomonad type of cell characteristically possesses a 

 single basin-shaped plastid, a red eye-spot, one pyrenoid and two 

 flagella, and is often strongly attracted to light (phototactic). 

 Variations in the structure of the cell occur throughout the genus, 

 which contains about 325 species. Some species lack pyrenoids (C. 



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