THE ALG.E 



227 



These protrusions finally meet and unite to form tubes connecting 

 the cells of the conjugating filaments in pairs (6). When the cell 

 wall between the united protrusions is absorbed, the protoplasts 

 of each pair of cells come into immediate contact, which initiates 

 in them certain changes resulting in their conversion into the 

 female and male gametes. This conjugation of the filaments 

 thus marks the initial stage of sexual reproduction in Spirogyra. 



G-amete formation is a very simple process consisting in the 

 shrinkage of entire 

 protoplasts in the 

 cells of two filaments 

 united by conjugat- 

 ing tubes. Each 

 gamete thus con- 

 sists of the original 

 cytoplasm, nucleus, 

 and chloroplast of an 

 ordinary Spirogyra 

 cell contracted into 

 a rounded mass of 

 protoplasm. In most 

 species of Sjoirogyra, 

 gametes are formed 

 in one of the conju- 

 gating filaments before the protoplast of the other filament begins 

 to contract. Since these first-formed gametes are the ones which 

 move through the conjugating tubes during fertilization to unite 

 with the stationary gametes of the adjacent filament, they are 

 usually considered to be the male gametes. Such species of 

 Spirogyra are therefore said to be dioecious, which means that 

 one filament bears only male gametes and the other only female. 



Gametangium is a term applied to the entire cell in which 

 the protoplast contracts to form a gamete. This cell may be 

 termed either the male or the female gametangium, according 

 as it forms a male or a female gamete. 



Fertilization consists in the union of the motile male gamete 

 with the stationary female gamete of the opposite filament. In 



FIG. 119. Sexual reproduction in Spirogyra 



a, early conjugation of the filaments (the pairs of uniting 

 cells are gametangia in each case) 1^6, gametes forming 

 and uniting in conjugation to form zygotes; c, zygotes 

 with thick cell wall, pyrenoids, and conjugate nucleus; 

 d, germination of the zygote to form a new plant 



