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 chemical differ- 

 ence between them. It has been 

 observed that there are two 

 types of hyphae, which have 

 been named the i)lus 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, 

 hj^phae 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- 



, 1 1111 .1 . • Hyphae of mold plants grow toward each 



solved, probably by the action other and meet, in the area of contact, mate- " 

 i? nni J. I- T, ^1 rial from each intermingle to form a zygospore. 



or enzymes. I hen through the 



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 pass 



WH. FITZ. AD. BIO. — 18 



.J^gos^pore 



