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, nor would it be any good if 

 they could, for without light assimilation cannot go on, and 

 therefore life is impossible for a green plant. 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 Ulotlirix 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 an hour or two, 

 and when they come to rest attach themselves by the 

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

 filament, like the parent. 



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



