VOLVOCALES 



33 



by cytoplasmic threads passing from the central protoplast to the 

 cell wall. The protoplast contains several contractile vacuoles and 

 one or more pyrenoids, although two is the usual number. Asexual 

 reproduction is by means of two to four macrozoospores, whilst 

 sexual reproduction is isogamous or anisogamous. The young 

 Sphaerella cell is very akin to a Chlamydomonas, and for this reason 

 some authors would unite the two genera. Large akinetes are 



Fig. 23. Sphaerella lacustrh {Haematococcus pluvialis). A, diagram of single 

 macrozoid. fe = blepharoplast, c = chloroplast, cf = flagellum tube, cw = c&W wall, 

 n = nucleus, nM = nucleolus, ^ = pyrenoid, ^5 = protoplasmic strand, r = rhizoplast, 

 5 = stigma. B, encysted plant with haematochrome in centre. C, eight-celled 

 palmelloid stage. D, diagram illustrating life cycle in bacteria-free cultures. 

 (After Elliott.) 



known which on germination give rise to zoospores, hypnospores, 

 or gametes. One species forms one of the components of "Red 

 Snow" because under nival conditions it develops haematochrome 

 as a result of nitrogen deficiency brought about by the presence of 

 the snow. Periodic drying also appears to be an essential factor if 

 the life history of the common species, Sphaerella lacustris, is to be 

 maintained. Eight-celled colonies (coenobia), which behave just 

 like Pandorina, are known in the related genus Stephanosphaera. 



Tetrasporaceae : Tetraspora {tetra, four; spora, spores). Fig. 24. 

 The members of this genus form expanded or tubular, convoluted, 



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CSA 



