266 



SEXUAL REPRODUCTION 



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 

 differentlv. 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 seem to be two different strains of bread mold 

 called plus and minus. The hypha of unlike strains attract 

 each other and zygospores are formed. 



^^-^rry .^ * . «t' 



When two threads of Soirogyra 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. 



