SEXUAL REPRODUCTION. 



275 



pear-like (E, fig. 301). The protoplasm at the narrower end 

 is more transparent and bears two or more 

 cilia ; while the larger end is occupied by 

 the reserve food and particularly the chloro- 

 plasts, if present. Union of free motile 

 gametes occurs by gradual coalescence, be- 

 ginning at the pointed, transparent end (F, 

 fig. 301). When the conjugation is com- 

 plete the resulting spore (zygote) usually 

 acquires a spherical form, soon secretes about 

 itself a wall, and either begins to grow at 

 once into a new plant or thickens the wall 

 and becomes dormant for a time as a resting 

 spore. In other cases the form of the 

 gametes is determined only by the shape of 

 the cell, from which they do not escape. The * 

 entire cell contents constitutes the gamete 

 (figs. 303, 304). In such plants both 

 gametes may be equally motile and meet 

 in a branch, the conjugating tube, to form a 

 spore, as in Mesocarpus (fig. 304) ; or the 

 male gamete may be motile and migrate from the cell in 

 which it is produced, through the conjugating tube into the 

 cell containing the female gamete, with which it fuses, as in 

 Spirogyra (fig. 303). 



The spore thus formed may be a resting spore, in which 

 case it secretes about itself a thick wall, and remains dormant 

 for several weeks or months. In the plants just referred to, 

 the spores, formed in early summer, with the remnants of the 

 parent cell-walls about them, sink to the bottom of the water, 

 and do not germinate till the next spring. 



contents of the two 

 upper cells are accu- 

 mulating in the con- 

 jugating tube to form 

 a zygote, which is 

 complete in the 

 lower tube. Magni- 

 fied about 150 diam. 

 —After DeBary. 



