162 STRUCTURAL BOTANY 



of light. In darkness these zoospores wander aimlessly 

 about in all directions, and do not come to rest until 

 they are exhausted and die. In darkness therefore they 

 cannot reproduce the plant ; but as they are normally 

 set free in the morning, and come to rest before evening, 

 this difficulty does not arise in Nature. Light, how- 

 ever, shining from one side, at once exercises a directive 

 effect on the movements of the zoospores. If the light be 

 of moderate intensity, they swim towards its source; if the 

 brightness be excessive, they hasten with equal decision in 

 the opposite direction. As the swarming period nears its 

 end, the zoospores tend more and more to avoid the light, 

 and so are brought into contact with dark solid objects in 

 the water, on which they can come to rest and germinate. 

 Hence the reaction to light is of service to the plant, for 

 the zoospores are thus induced to disperse themselves in 

 the open water where their own assimilation can go on, 

 while at the same time seeking shelter from too intense 

 sunlight, which is dangerous to unprotected protoplasm. 

 Finally, their changing sensitiveness ultimately brings 

 them to rest in a position where germination can 

 take place. 



The zoospores of Ulothrix and of many other Algae are 

 provided with a red pigment-spot at one side of the 

 clear protoplasm ; this has been called the eye-spot, a 

 fanciful name, though it is possible that the red pigment 

 may really have something to do with the action of light 

 on the zoospore. These comparatively large, four-ciliated 

 zoospores of Ulothrix swim about for some hours, and 

 when they come to rest attach themselves by the colour- 

 less end, and grow out at once into a new Ulothrix 

 filament, like the parent. 



In other cells, it mav be of the same or of different 



