THE DEVELOPMENT OF SEXUALITY IN PLANTS 



171 



the tube and fuse with th(^ quiescent cells. When this fusion takes 

 place, the nuclei unite so that a single resting cell is formed, called the 

 zygote, which develops a thick wall, very resistant to drought and cold. 

 The zygote is heavy enough to sink to the bottom of the pond when 

 the rest of the filament dies, and under favorable conditions will 

 germinate, giving rise to a new filament. 



Since these cells from different filaments join or fuse, somewhat 

 after the manner of conjugation in Paramecium, we think of them as 

 sex cells, or gametes. Although the two cells are of the same size, 

 yet one is active and the other passive. In higher plants and animals, 

 the active cell is referred to as the male gamete, or spei^m, and the 

 non-active cell as 

 the female gamete, 

 or egg. A compari- 

 son of Spirogyra 

 with higher forms 

 suggests a very sim- 

 ple type of sexual 

 reproduction, known 

 as conjugation. 



In another fila- 

 mentous form, Ulo~ 

 thrix, certain cells 

 are modified to be- 

 come free-swimming 

 zoospores, provided 

 with four cilia which 

 may swim about for 

 as long as an hour 

 before settling 

 down. It is obvious 

 that such a free- 

 swimming cell may plant a new individual at some distance from 

 the original filament. Gametes of Vlothrix are also formed as free- 

 swimming cells, all alike, having two cilia instead of four. These 

 gametes fuse by conjugation and produce a zygote, which, like that 

 of Spirogyra, has a thick resistant wall, and is capable of developing 

 even after exposure to very unfavorable conditions. 



In the formation of the conjugating gametes of both Vlothrix and 

 Spirogyra a significant thing happens to the nuclei of the cells before 



Ulothrix: a. base of filament with holdfast; b, fila- 

 ment producing; zoospores or gametes; c. young filament 

 developed from zoospore; d. filament discharging zoo- 

 spores and gametes ; e. an escaped zoospore ; /, escaped 

 and pairing gametes ; g, zygospores ; h, zygospore pro- 

 ducing zoospores by reduction division, (a-f/. After 

 Coulter; //, after Dodel-Port.) 



