34 THE ALGAE 



eye-spot is photosensitive it is possible that light intensity deter- 

 mines this initial arrangement. 



Recently some interesting information has come to light con- 

 cerning the nuclear condition of the Volvocaceae (Cave and Pocock, 

 195 1). Gonium, which would seem generally to be the most primi- 

 tive member of the family, possesses the largest number of 

 chromosomes (n = 17). In Volvox the species regarded as primitive 

 have more chromosomes (n = i4 or 15) than species regarded as 

 advanced (n=5). The facts may involve a re-interpretation of 

 development within the family, Gonium being regarded as reduced 

 rather than primitive, and similarly with the species of Volvox, 

 In the genus Astrephomene colonies with 4, 6, 7 or 8 chromosomes 

 occur. Polyploidy is suggested since clones with 4 chromosomes 

 remain distinct from those with 6, 7 or 8. 



Tetrasporaceae: Tetraspora (tetra, four; spora, spores). Fig. 12 



Members of this and allied famihes are placed by some workers 

 in a separate order, the Tetrasporales. They are characterized by 

 non-motile vegetative cells that may temporarily become motile. 

 In so far as the cells are typically chlamydomonad-like, there seems 

 no justification for placing the genera in a separate order. The 

 species of Tetraspora form expanded or tubular, convoluted, light 

 green macroscopic colonies. The colony in these palmelloid forms 

 differs from the organized coenobium of the Volvocaceae because 

 it is merely an unorganized loose assemblage of cells. The palmel- 

 loid condition, which occurs from time to time in members of the 

 Chlamydomonadaceae, has in this and alUed genera become the 

 dominant phase, whilst the motile condition only occupies a brief 

 period in the Ufe cycle. 



The colonies are most abundant in the spring when they are 

 attached at first, although later they become free-floating. The 

 spherical to ellipsoidal cells are embedded in the mucilage and are 

 frequently arranged in groups of two or four, each group often 

 being enclosed in a separate envelope. Two pseudociUa proceed 

 from each cell to the surface of the main colonial envelope, each 

 thread being surrounded by a sheath of denser mucilage. These 

 structures cannot be organs of locomotion because there is no 

 power of movement, but they either represent such organs which 

 have lost their function or they are their precursors. 



Vegetative reproduction takes place by fragmentation of the 



