102 THE CELL 



amongst themselves, keeping more or less close to the edge of 

 the drop, according to the condition of the preparation. Continu- 

 ally, individual spores are seen to suddenly forsake the side, either 

 positive or negative, at which they were stationed, and to hurry 

 through the drop to the opposite one. Such an exchange is 

 continually taking place between the two sides. Indeed, it 

 frequently occurs that certain individuals, which have just left 

 one side and arrived at the other, hasten back to the one from 

 which they originally came. Others become arrested in the middle 

 of their course, and then return to their starting-point, in order 

 eventually to oscillate backwards and forwards for a considerable 

 time like a pendulum." 



The following experiment, described by Strasburger, shows 

 how sensitively and quickly the zoospores react to light : " If a 

 piece of paper is placed between the microscope and the source of 

 light, just as the zoospores are on their way from one edge of the 

 drop to the other, they immediately turn to one side, many 

 rotating in a circle ; this, however, only lasts for a moment, 

 after which they continue to move in the same direction as before 

 (interruption movements)." Strasburger (IV. 37) has named those 

 zoospores ivhich hasten towards the source of light light -seeking 

 (photophylic), and those which travel from it light -avoiding {photo- 

 phobic). 



As has been already remarked, the way in which the spores 

 collect at one or other side of the drop, thus indicating their 

 special hind of phototonus, depends upon external circumstances, 

 such as the intensity of the light, the temperature, the aeration of 

 the water, and their condition of development. 



It is possible to entice spores, which under intense illumination 

 have collected on the negative side, to come over to the other side. 

 The intensity of the light must be gradually diminished in pro- 

 portion to their phototonus by introducing one, two, three or more 

 screens of ground glass between the preparation and the source 

 of light. The same result may be more easily obtained by moving 

 the microscope slowly away from the window, and thus rendering 

 the illumination less intense. 



The temperature of the environment often has a considerable 

 influence upon the degree of sensitiveness to light which is evinced 

 by many zoospores. When the temperature is raised they become, 

 so to speak, attuned to a greater degree of sensitiveness ; whilst, 

 at the same time, their movements are rendered more active : the 



