THALLOPHYTES 



29 



active gametes are male, then the emptied filament is a male 

 individual, and the receiving filament is a female individual. 

 In such a case, there- 

 fore, there is a sexual 

 differentiation of indi- 

 viduals, and in Spiro- 

 gyra this occurs with- 

 out any differentiation 

 of gametes in appear- 

 ance, and without any 

 differentiation of sex- 

 organs. 



After the origin of 

 sex, therefore, when 

 the formation of gam- 

 etes is an established 

 habit, there are three 

 kinds of differentia- 

 tion : differentiation 

 of gametes, of sex- 

 organs, and of sex- 

 individuals. These 

 different kinds of dif- 

 ferentiation may occur 

 singly, or any two to- 

 gether, or all three to- 

 gether. When the last 

 takes place, and we 

 find plants with eggs 

 and sperms, produced 

 by distinctly set apart 

 oogonia and anther- 

 idia, and these two 

 kinds of sex-organs borne on different individuals, we have 

 reached an extreme case of sexual differentiation. 



FIG. 22. Spirogyra: A, part of a filament, show- 

 ing one complete cell, with its central nucleus 

 and its characteristic chloroplast (the spiral 

 band) ; B, cells of two filaments developing the 

 connecting tubes ; C, the passage of one pro- 

 toplast through the tube ; D, the oospore 

 formed by the fusing of the two protoplasts; 

 the emptied cell is therefore male and the cell 

 containing the oospore is female, and if all the 

 cells of each filament are like the one shown, 

 the filaments (individuals) are male and female. 



