144 BEHAVIOR OF THE LOWER ORGANISMS 



the optimum they are positive; in light above the optimum they are 

 negative. 



Strasburger did not determine the precise movements of the organ- 

 isms in the reaction to light. That is, he did not determine toward 

 which side they turn in becoming oriented. But in other respects his 

 account is so excellent that, with the fuller results on Euglena as a key, 

 it is not difficult to analyze out the precise factors in the behavior. 



If the light affecting the organisms is suddenly decreased in intensity, 

 Strasburger found that the swarm spores (Botrydium and Ulva) sud- 

 denly turn toward one side (I.e., p. 25). In Bryopsis this reaction was 

 produced also when the light was suddenly increased. In all the swarm 

 spores it was evident that as soon as the light was decreased by the in- 

 terposition of a screen the path became more crooked (I.e., p. 27). 

 In other words, the spiral became wider, owing to the increased swerv- 

 ing toward a certain side. In these respects the swarm spores precisely 

 resemble Euglena. It is clear that they react to a sudden decrease in 

 illumination by an avoiding reaction, which consists in turning more 

 or less strongly toward a certain side, with or without a cessation of the 

 revolution on the long axis; in this way the direction of progress is 

 changed. 



As would be expected from this method of response, the organisms 

 react at passing from a light to a dark region. If a ring is placed over 

 the drop containing the organisms, so that only a central circle is illu- 

 minated, the positive organisms gather in the illuminated circle (Fig. 

 98, A). Here they swim toward the window from which the light 

 comes, but on reaching the edge of the shadow, they turn back into the 

 lighted region (I.e., p. 28). Often the organism passes completely 

 into the shadow before reacting, then it turns and swims back into the 

 light. Thus it does not react till a short time after the moment of change. 

 If a narrow band of shadow passes across the middle of the drop, trans- 

 versely to the direction from which the light is coming, this usually does 

 not stop the organisms, because of this interval of time which elapses 

 before their reaction ; before they begin to react they have passed com- 

 pletely across the band into the lighted region beyond. But if a larger 

 vessel is used and a broader transverse band of shadow passes across 

 it (Fig. 98, B), this does stop the organisms. They gather on the edge of 

 the shadow without passing across it. In many other ways Strasburger 

 shows that when the area containing the swarm spores is unequally 

 illuminated, the positive organisms collect in the more illumined region. 

 In this they precisely resemble Euglena, as Strasburger himself noted. 

 The behavior is of course a direct result of the production of the avoid- 

 ing reaction by a decrease in light. 



