SEXUAL REPRODUCTION IN SPIROGYRA 



265 



zygospore may be defined as a structure resulting from the conju- 

 gation of similar gametes. Although the hyphae of a mold all look 

 somewhat alike, there must be a physiological or cjiemical differ- 

 ence between them . It has been 

 observed that there are two 

 types of hyphae, which have 

 been named the plus and the 

 minus strains. If a plus strain 

 meets a minus strain, conju- 

 gation will occur. It is now 

 possible to isolate or to pur- 

 chase plus and minus strains 

 of mold spores. When these 

 are planted on opposite sides 

 of a slice of moistened bread, 

 hyphae will grow out from each 

 strain and when they meet, 

 zygospores are formed. 



Sexual reproduction in Spiro- 

 gyra. During sexual reproduc- 

 tion in the Spirogyra, portions 

 of the cell walls between the 

 two filaments grow perpendic- 

 ular projections and form a 

 bridge. The cell walls in the 

 middle of the bridge are dis- 

 solved, probably by the action 



of enzymes. Then through the rkl from each intermin * le to form a 

 work of vacuoles the entire contents of the cell in one filament are 

 moved across the bridge and fused with the protoplasm of the 

 other cell. After conjugation, a thick cell wall develops around 

 the fused protoplasm and the structure is known as a zygospore. 

 If one cell in a filament is an active cell, that is, its contents t pass 



WH. FITZ. AD. BIO. 18 



