TROPISMS AND RELATED PHENOMENA 153 



state of contraction, and the animal will bend or move in the direction 

 of the lines of diffusion. There is practically, however, this difficulty; 

 namely, that the lines of diffusion are generally disturbed by currents 

 due to changes and variations in temperature, and instead of the straight 

 lines of force we have in this case often irregular and changing ones. 

 This makes it a priori hopeless to expect that in the case of chemo- 

 tropism the organisms move in as straight a line as in that of heliotro- 

 pism, geotropism, and certain cases of galvanotropism. In the majority 

 of cases we are also dealing with a response to sudden changes in the 

 nature and concentration of the substances contained in the medium. 



Engelmann was probably the first to call attention to this type of 

 phenomena. He found that certain bacteria and Infusorians gather 

 around a source of oxygen.* In this case it was evidently a response 

 to changes in the concentration of the oxygen, the organisms coming 

 to rest where the tension of oxygen was a relative maximum. Pfeffer 

 first compared these phenomena with those of other tropisms in his 

 classical paper "Oriented Locomotor Motions produced by Chemical 

 Stimuli." f He showed that the zoospermiae of ferns, mosses, and other 

 plants move toward points from which certain substances diffuse into 

 the water in which these organisms are. Pfeffer found that such effects 

 are produced upon spermatozoa of ferns by malic acid and its salts, 

 and upon those of mosses by cane-sugar solutions. The biological 

 importance of this observation lies in the fact that malic acid is com- 

 paratively common in plants, and the presence of this acid in the arche- 

 gonia of the ferns possibly contributes toward bringing the sperm to 

 the egg. From the normal archegonium no malic acid diffuses, but 

 those ready to be impregnated let part of their contents diffuse. The 

 appearance of Pfeffer's paper aroused in many the hope that it might 

 be shown that the animal egg, too, attracted the spermatozoa in some 

 such chemotropic or chemotactic way; but all the experiments thus 

 far made in this direction by J. Dewitz, Buller, and others - - 1 have 

 made quite a few experiments myself on this subject - - have without 

 exception shown that such is not the case in the eggs thus far tried. 

 There does not seem to exist an attraction of the spermatozoa on the 

 part of the egg, but the meeting of spermatozoa and the egg is left to 

 chance, except that automatic tropismlike mechanisms exist, whereby 

 the ripe males gather near the ripe females and the sperm is shed in the 

 neighborhood of the egg. Pfeffer's method consisted in introducing 

 a solution of the chemical substance to be tested, e.g. malic acid, into 



* Engelmann, Pfliiger's Arckiv, Vol. 25, p. 285, 1881; Vol. 26. p. 537, 1881 ; and 

 Vol. 29, p. 387, 1882. 



t Pfeffer, Unters. aus dem bot. Institut in Tubingen, Vol. I, p. 363, 1881-1885. 



