IRRITABILITY 235 



of decaying matters, attracted thither by the substances 

 diffusing from the dead organic material. The sensitiveness 

 of the bacteria of disease to the dissolved substances in the 

 body of their host enables them to move toward and into 

 the places most favorable to them. The chemotactic sensi- 

 tiveness of bacteria, flagellates, Yolvocinsie, and motile 

 reproductive bodies was studied very thoroughly by Pfeffer. * 

 His method consisted in the employment of capillary tubes 

 closed at one end and partly filled under the air-pump 

 with solutions of the substances to be tested. These tubes, 

 after rinsing, he introduced into drop-cultures or under 

 the coverglass on the slide. From the mouths of such tubes 

 the molecules of the dissolved substances diffuse in all direc- 

 tions into the surrounding liquid and presently impinge 

 upon the motile organisms present. Such impinging mole- 

 cules set up an irritation within the cell. When this irrita- 

 tion is sufflcient, the organism reacts by changing its posi- 

 tion, moving so that its long axis becomes parallel to the 

 line of advancing molecules, and presently going either to- 

 ward or away from the source of the irritating substance, 

 according as, this substance attracts or repels. Pfeffer 

 found that certain substances attract, others repel, and still 

 others induce no change in the direction of locomotion. 

 Pfeffer's list has been considerably lengthened by one of his 

 pupils, BuUer, who worked on the antherozoids of ferns, t 

 As a result of Pfeffer's work it is now generally accepted 

 that the antherozoids of ferns, liverworts, and algte make 

 their way to the egg-cells under the definite attraction of 

 soluble substances diffusing from the egg-cells themselves, or 

 from a part or the whole of the oogonium or archegonium. 

 The specifically attractive substances are unknown, though 

 it is inferred in the case of ferns that malic acid is the at- 



* Pfeffer, W. Locomotorische Richtungsbewegungen durch chemische 

 Beize. Unters. a. d. bot. Inst. z. Tubingen, Bd. I., 1884. f)ber chemo- 

 taktische Bewegungen von Bakterien, Flagellaten und Volvocineen. ibid. 

 Bd. II., 1888. See also Jennings and Crosby on manner in which bacteria 

 react to stimuli, especially to chemical stimuli. Amer. Journ. Physiol., 

 vol. VI., 1901. 



t BuUer, A. H. R. Contributions to our knowledge of the physiology of 

 the spermatozoa of ferns. Annals of Botany vol. 14, 1900. 



