4i6 



How Aiiwiah and Flmns Reproduce unit viii 



Fig. 365 Two filainents of conjugating Spirogyra. Conjiigatiojj is complete in three 

 pairs of cells. In one the gaynete from the upper filament is passing into the lower. 

 Which is the latest stage? (ward's natural science establisHxMent) 



cell in a filament of Spirogyra is a gamete. 

 The cell that flows across the bridge into 

 the other cell is called an active or sup- 

 plying gamete; the one which remains 

 within its cell wall is the passive or re- 

 ceiving gamete. After the two gametes 

 have united, the filament which con- 

 tained the active gametes is left with 

 empty cell walls. The other filament, 

 which originally contained the passive 

 gametes now contains all the fused masses 

 of protoplasm. Each fused mass of proto- 

 plasm loses water, shrinks, and secretes 

 a heavy wall around itself. It is then 

 known as a zygospore (zeve'go-spore). 

 The zygospore, even though it is made 

 from the fusion of two cell bodies does 

 not even fill the original cell wall. See 

 Figure 365. This cell wall gradually de- 

 cays and then the zygospore sinks to the 

 bottom of the pond. 



But the story of reproduction in Spiro- 

 gyra is not yet complete. So far, two 

 cells have united, forming one. The sec- 

 ond chapter of this story may take many 

 weeks to complete, for the zygospore 

 may rest for a long time. Because of its 



heavy wall it is usually not killed even if 

 the pond dries up. When conditions be- 

 come favorable, the living protoplasm 

 within the zygospore wall becomes ac- 

 tive. It absorbs water and bursts open its 

 covering. The chloroplast manufactures 

 sugar, the cell absorbs water and min- 

 erals, makes proteins, and grows. When 

 the cell has grown to normal size first the 

 nucleus and then the cell body divides. 

 After repeated divisions there is a whole 

 new filament or Spirogyra plant. The 

 process of reproduction is now complete. 

 This second step usually takes place in 

 the spring. Prepared slides that show the 

 various stages in the fusion of Spirogyra 

 gametes can be bought, or you may be 

 lucky enough to collect the plant in just 

 the right stage. See Exercise 6. 



In this method of reproduction the 

 new individual is formed only after the 

 fusion of the two cells. TIius reproduc- 

 tion in Spirogyra is difl^erent from the 

 methods described earlier in this prob- 

 lem. There are many other orcfanisms in 

 which there is a fusion of gametes from 

 two separate individuals. The common 



