HISTORICAL REVIEW I 5 



the shadow produced by the side of the dish. This fact led 

 some authors to conclude that these organisms avoid the 

 light, but this did not account for the fact that the swarm 

 spores collect also at the window side of a dish which pro- 

 duces no shadow and in which this part is most highly 

 illuminated. Cohn recognized this difficulty and con- 

 cluded in 1865, eleven years before Sachs announced his 

 ray-direction theory, that it is not difference of intensity 

 in the field but direction of the rays that regulates the 

 direction of movement in these organisms. He does not, 

 however, make it clear whether he means direction of the 

 rays through the tissue or direction in the field. 



Sachs answered the question as to the cause of aggre- 

 gation in unicellular forms in a very simple way. He found 

 (1876, p. 241) that certain inanimate particles suspended 

 in water collect in definite regions when exposed to light 

 owing to currents caused by variation in temperature. 

 He was of the opinion that the movem.ent and aggregation 

 of unicellular forms under similar conditions were largely 

 if not entirely of the same nature. 



For the express purpose of testing this opinion, Stras- 

 burger (1878, p. 552) studied the reactions of swarm spores 

 to light. He repeated the experiments of Sachs and ob- 

 tained confirmatory results, but concluded from detailed 

 microscopic observations on the movements of these organ- 

 isms that the aggregations formed in light under normal 

 conditions are almost entirely due to active swimming of 

 the swarm spores and not to currents in the water. Stras- 

 burger in this paper, however, incidentally supports the 

 general theory of Sachs on heliotropism. He found in 

 agreement with Nageli's observation (i860) that positive 

 swarm spores move toward a source of light even if in so 

 doing they pass from regions of higher light intensity into 

 regions of lower, and concluded just as Cohn (1865) had, 

 that this cannot be due to difference of intensity. He does 

 not however consider the fact that under the conditions of 

 his experiments the anterior ends of the spores were con- 



