CHAPTEK VIII 



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 w r e 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, str. 

 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 convenient 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. 



The fully developed Spirogyra plants con- 

 sist 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. ?i-.), whereby its protoplasmic contents are 

 completely separated from those of adjacent 

 cells. The cytoplasm forms a thin primordial 

 utricle (p.w.), 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 



FlG. 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 ; nu., 

 nucleus ; p.u., pri- 

 mordial utricle; 

 Py r -> pyrenoid; sir., 

 strands of proto- 

 plasm; vac., vacuole. 



