578 BIOLOGICAL EFFECTS OF RADIATION 



single contact and that this is associated with marked increase in the 

 thickness of the plasmagel at the tip, and he concludes that a series of 

 weaker contacts induces reversal, owing to cumulative gelating effect, 

 and that we therefore have here a fairly comprehensive insight into a 

 process Avhich closely resembles what is usually called "learning." 



FLAGELLATES 



A large number of the different flagellates respond to light and many 

 of them respond very precisely. The response has, however, been 

 thoroughly investigated in only a few species, but it is probably funda- 

 mentally the same in all. 



EUGLENA 



Euglena either swims freely through the water by means of the action 

 of its flagellum or it creeps on the substratum without the action of the 

 flagellum, but it always rotates on the longitudinal axis as it proceeds. 

 If the intensity of the light is rapidly changed, the organism responds 

 by abrupt change in rate and in direction of movement. Under some 

 conditions it responds to increase and under others to decrease in inten- 

 sity. If the intensity is slowly changed it does not respond, at least not 

 in this way. This is a typical shock-reaction. 



The response to decrease in intensity results in aggregation in brilliant 

 spots in a field of light; the response to increase results in aggregation 

 in dark spots. These spots act like a trap; the Euglenae get into them 

 by random movement, but when they reach the boundary on the way 

 out, the change of intensity of the light induces the shock-reaction and 

 prevents their exit (Engelmann, 58). 



In a beam of light, Euglena usually orients very precisely and goes 

 either toward or from the light. Verworn (207) postulates that when 

 it is not oriented, and opposite sides are unequally illuminated, the 

 flagellum beats more effectively in one direction than in the other, that 

 this results in turning until opposite sides are equally illuminated, and 

 that the flagellum then beats equally in opposite directions, and the 

 organism continues on a straight course. 



This theory is in principle essentially the same as that formulated 

 by Ray in 1693, in reference to orientation in plants, and accepted by 

 De Candolle (31) in 1832. According to it the effect of light on the 

 activity of the motor mechanism is dependent upon the intensity (not 

 upon change of intensity) of the illumination of this mechanism or the 

 photoreceptors connected with it. The light acts continuously after 

 orientation has been attained, as well as during the process of orientation, 

 the only difference being that during the process of orientation the 

 illumination on opposite sides is unequal, and consequently results in 

 quantitatively unequal action in the motor mechanism; while after 



