THE VITAL PROPERTIES OF THE CELL 101 



This sensitiveness towards light varies considerably both in 

 different species and in individual members of the same species ; 

 indeed, even in the same individual, considerable differences may 

 be seen under different external conditions. This varying power 

 of reaction in swarm-spores has been called phototonus or light- 

 tone by Strasburger. 



Swarm-spores of the Botrydium and Ulothrix, which react some- 

 what differently under the influence of light, are very suitable 

 for experiments on this subject. 



If some swarm-spores of Botrydium are placed in a drop of 

 water upon a coverglass, and are kept in shadow, they spread 

 themselves out evenly in the water. If a light is allowed to fall 

 on them, they are seen to immediately direct their anterior ends 

 towards the source of light, and to hurry in fairly parallel paths 

 towards it. After a short time, at most from one and a half 

 to two minutes, almost all of them have collected at the illuminated 

 side of the drop, which, for the sake of brevity, Strasburger has 

 named the positive edge, to distinguish it from the opposite or 

 negative edge. Here they are seen to intermingle and to conjugate 

 in large numbers. If the slide is now turned round through an 

 angle of 180, all the spores which are still capable of movement 

 immediately forsake the edge of the drop, which is now turned 

 away from the light, and hasten back towards the light. If 

 the microscope is fitted with a rotating stage, it is possible by 

 turning the latter to make the swarm-spores continually keej} 

 changing their course. They always travel in a straight line 

 towards the light. 



Ulothrix zoospores behave in a somewhat different manner. 

 " These also travel quickly, and in approximately straight paths 

 towards the positive edge of the drop ; however, as a rule, they do 

 not all move in this manner; on the contrary, it is generally the 

 case that a larger or smaller number of individuals in each prepara- 

 tion are seen to move rapidly in the opposite direction, that is 

 to say, towards the negative edge. A most peculiar spectacle is 

 thus produced, for the spores, since they go in opposite directions, 

 appear to travel at double speed as they pass each other. If the 

 preparation is turned through an angle of 18l>, the spores which 

 had collected on the side which was positive are seen to hasten 

 to the other edge, whilst the others, which were collected on the 

 side which was negative, travel in the opposite direction, and 

 having arrived at their destination, they commence to move about 



