STIMULI AND THEIR ACTIONS 



449 



toward the source of light, and betake themselves to the opposite 

 or negative side ; by further increase of the intensity all collect at 

 the latter side. There exists, therefore, a point in the intensity, 

 toward which the swarm-spores rush, going toward it from both 

 higher and lower intensities a phenomenon that Strasburger 

 termed photometry. There is here a complete analogy to 

 chemotaxis ; the latter is positive up to a certain concentration of 

 the effective substance, but from there on with increasing con- 

 centration is negative, so that the term chemometry is justified. 

 Quite analogous to the behaviour of the swarm-spores of Ulothrix 

 is that of the swarm-spores of Chcetomorpha, Ulva, Hcematococcus, 

 and some other Algae, as well as the flagellate infusorian Chilo- 

 monas Paramcecium, and the colourless swarm-spores of the 

 Chytridice, all of which are 

 positively phototactic with 

 feeble intensity of light, and 

 negatively phototactic with 

 stronger intensity. There 

 are forms e.g., the swarm- 

 spores of Botrydium grami- 

 latum which show positive 

 phototaxis in all intensities. 

 Next to these researches 

 of Strasburger comes a 

 whole series of observations 

 by other investigators, who 

 have been able to find 

 phototactic phenomena in 

 all sorts of micro-organisms. 

 Thus, Stahl ('84) investi- 

 gated the phototaxis of 

 plasmodia of Myxomycetes, 

 previously observed by Hof- 

 meister and Baranetzky, and 

 found that young plasmodia 

 of Aethalium septicum are 



positively phototactic in half-darkness, and creep upon the surface 

 of tan, but with stronger illumination they become negatively photo- 

 tactic, and flow back again into the interior of the rhass. Further, 

 Engelmann ('81, 3; '83) found mBacterium chlorinum and Bacterium 

 plwtometricum two forms that possess phototactic properties and 

 collect together in the light. Engelmann ('82), Stahl ('80), Aderhold 

 ('88), and others x discovered phototactic phenomena also in the 

 Diatomece and the Oscillarice, which behave exactly as the swarm- 

 spores of Alyce and form very pronounced assemblages (Fig. 223). 

 Finally, Stahl (/.c.), Klebs ('85), and Aderhold (I.e.) demonstrated 

 1 Cf. Verworn ('89, 1). 



G G 



FIG. 223. Phototaxis of Diatomece. A particle of mud 

 which was thickly surrounded by Diatomece lies 

 in the middle of the drop. The organisms have 

 all crept toward the edge turned toward the sun. 



