146 



BEHAVIOR OF THE LOWER ORGANISMS 



ing from one region to another was not sufficiently marked to cause 

 reaction. 1 



When the entire area is equally lighted and the light comes from 

 one side, the positive swarm spores swim toward the source of light. 

 If the light is made strong, most species swim away from its source. 

 In this behavior the agreement with Euglena is complete. The orien- 



FIG. 99. Diagram of the conditions in Strasburger' s experiments with a wedge-shaped 

 prism, constructed from the data furnished by Strasburger. a, prism 20 cm. in length, filled 

 with a translucent fluid, b, hanging drop containing the swarm spores. X, rays of light 

 coming from above, as in the first experiments. Y, rays coming obliquely from the thicker 

 end of the wedge, as in the second set of experiments. The figure is one half natural size. 



tation takes place gradually, by a series of trials, as in Euglena. Stras- 

 burger paid no special attention to this point, but the present writer 

 has observed that this is true in Cryptomonas, Chlamydomonas, and 

 the swarm spores of the marine alga Cutleria, as well as in Euglena, 

 and Strasburger (1878, p. 24) notes incidentally that it is true in Haema- 

 tococcus. 2 



It seems clear, then, that the reaction takes place in the same manner 



1 It is curious that Strasburger drew from this experiment the erroneous conclusion 

 that variations in the intensity to light play no part in the reaction. The only essential 

 difference between this experiment and the previous ones (Fig. 98) is that in the pre- 

 vious experiments the change of illumination in passing from one region to another 

 is sudden and pronounced, while in the present experiments it is slow and gradual. 

 The logical conclusion is that the lack of reaction in the present experiment is due to 

 the slightness of the change in passing from one part of the preparation to another. 

 When we consider that the prism was 20 cm. in length, and was placed over a mere 

 drop, it is evident that the difference in illumination in different parts of the drop 

 was excessively small. We know that for the effective action of all stimuli a certain 

 threshold amount of change is necessary, so that the results are exactly what might 

 be anticipated. Our account of Euglena shows beyond doubt that a change in intensity 

 of illumination does cause reaction. Strasburger himself (I.e., p. 25) observed the same 

 fact in swarm spores, though he paid little heed to this observation in the remainder of 

 the work. 



2 He says that when the direction of the light is changed, the swarm spores become 

 oriented " Nach verschiedenen Schwankungen." 



