266 



SEXUAL REPRODUCTION 



DD1 



through the bridge, all 

 the cells in that fila- 

 ment are active cells. 

 If one individual in a 

 filament is a passive 

 cell, all other cells of 

 that filament are pas- 

 sive or receiving cells. 

 There is more physio- 

 logical differentiation 

 in the Spirogyra than 

 in the mold, in that 

 the gametes behave 

 differently. The ac- 

 tive gametes may be 

 compared to male gam- 

 etes of higher plants and animals, the passive, or receiving gam- 

 etes, to female gametes. After the zygospores are formed, the 

 filament sheath breaks down, the cell walls disintegrate, and the 

 zygospores fall to the bottom of the pond. They stay in a dor- 

 mant condition until there is sufficient water and warmth to pro- 



There seems to be two different strains of bread mold 

 called plus and minus. The hypha of unlike strains attract 

 each other and zygospores are formed. 



When two threads of Spirogyra lie parallel to each other, ad- 

 jacent cells may send out little tubes which meet. The cross 

 walls in the tubes become dissolved, leaving an unobstructed 

 bridge. The entire contents of one cell (active gamete) will pass 

 over and mingle with the contents of the other cell (passive gam- 

 ete). Fusion takes place and a zygospore is formed. If one cell 



in a filament has a moving gamete, the contents of all the cells of that filament behave in 



a similar way. 



