CHAPTEK VIII 



str. 



Sexual phenomena in multicellular plants The distinction between somatic 

 cells and germ cells Alternation of sexual and asexual generations 

 Suppression of the gametophyte in flowering plants. 



WHEN we consider the habit of colony formation which is so 



common amongst the Protophyta, and which we have discussed 



in the cases of Pandorina, Eudorina and 



Volvox, we see at once that it is impossible to 



draw any strictly logical distinction between 



such primitive forms and the true multi- 

 cellular plants or Metaphyta. The common 



fresh water alga, Spirogyra, for example, 



might be regarded either as a colony of single 



cells or as a very simple multicellular plant 



in which the constituent cells exhibit little 



or no differentiation amongst themselves. 



In any case it forms a very con ;cnient start- 

 ing point for the consideration of the sexual 



phenomena met with in Metaphyta, being in 



this respect actually in a much more primitive 



condition than either Eudorina or Volvox. 

 /\yThe fully developed Spirogyra plants con- 

 /Bist of long green filaments of hair-like 



dimensions, which float in loose slimy 



masses in clear fresh water. Each filament 



consists of a single row of cylindrical cells 

 ) placed end to end, each cell being enclosed in 



a thin, transparent wall of cellulose (Fig. 42, 

 ~ c.iv.), whereby its protoplasmic contents are 



completely separated from those of adjacent 



cells. The cytoplasm forms a thin primordial 



utricle (p. u.), lining the cell-wall and enclosing 



a large vacuole (vac.) filled with colourless, watery cell-sap, in 



which a more or less central mass of cytoplasm, containing the 



FIG. 42. Part of a 

 filament of Spiro- 

 gyra, showing 

 one complete cell 

 and parts of two 

 others ; highly 

 magnified. 



c.w., cell-wall; cr., chro- 

 matophore ; int., 

 nucleus ; p.u., pri- 

 mordial utricle; 

 pyr., pyrenoid; str., 

 strands of proto- 

 plasm; vac., vacuole. 



