ULOTHRIX ZONATA 41 



in any cell of a filament. In due time they are liberated 

 through a side opening, formed by absorption, of the 

 wall of the mother-cell. In addition to its four cilia, 

 which rise from the transparent point of the pear- 

 shaped body, each zoospore possesses a red pigment 

 spot and a single chloroplast. Another feature is the 

 " pulsating vacuole," a space in the cell which exhibits 

 alternate expansions and contractions every few seconds. 

 It has been demonstrated that zoospores are ineffectual 

 in darkness, and Nature sees to it that they are liberated 

 in the morning, so that they may have the benefit of 

 daylight for their operations. On their escape, they 

 swim about very actively, and might easily be mistaken 

 for animals. But they object to too brilliant a light, 

 and recede from it; moreover, as the end of their brief 

 but merry state of activity approaches, before the 

 descent of darkness, they show a strong disposition to 

 avoid light. This disposition secures their dispersal, 

 and so it happens that, after a few hours of activity, 

 they become dispersed, and settle down on some object 

 in the water on which they can germinate. In ger- 

 mination each zoospore produces a short process with 

 which it attaches itself to its substratum, and gives rise 

 to an ordinary Ulothrix filament. Thus we see that 

 reproduction by means of zoospores is asexual. 



The sexual gametes (see Fig. 12), which, as has been 

 noted, are usually smaller than the asexual zoospores and 

 have only two cilia, are produced in other cells of a fila- 

 ment ; they may number from four to thirty-two per cell. 

 Like the zoospores, they have one chloroplast, a red 

 pigment-spot, and a pulsating vacuole. On liberation 

 from the mother-cell they swim gaily in a swarm. It 



6 



