312 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS 



stimulation of the sperms of Ferns produced no acceleration of movement. 

 Nevertheless, if an organism whose movement is retarded or inhibited by 

 a deficiency of oxygen or of a food-material is tropically stimulated by the 

 unilateral access of oxygen or of the food- material, an acceleration of 

 movement is bound to ensue at the same time that the tropic response 

 is given 1 . Similarly, in the negatively photophobic Bacterium photometricum 

 the power of movement is first awakened by exposure to light. Various 

 tropic curvatures involve an acceleration or retardation of the average rate of 

 growth, and in the nodes of grasses the awakening of growth is due to the 

 geotropic stimulation which produces curvature. 



Although certain tropic movements may have a purely physical origin, 

 the reactions of plasmodia and of Amoebae are undoubtedly physiological 

 responses, although Rhumbler and Verworn 2 consider those of the latter 

 to be directly due to changes of surface-tension. The latter may act as 

 stimuli and may also play an important part in the performance of move- 

 ment, but nevertheless the fact that the amoeboid activity is shown in 

 homogeneous media indicates that it is under the control of the organism. 

 Hence the tactic movements of zoospores are no more to be regarded 

 as the direct result of a modification of surface-tension by the external 

 agency, than is the flying of a moth towards a candle or the curvature of 

 a plant towards light. 



The cilia of Chlamydomonas and of other forms appear thigmotropically 

 excitable, for a rapid general response is shown when only the tip of a 

 cilium is in contact with a foreign body. It is, however, uncertain whether 

 the cilia are the perceptive organs for chemotactic and other tactic stimuli. 

 Phototactic stimuli appear to be perceived neither by the cilia nor by the 

 eye-spot of Euglena^ but by its hyaline anterior end. In any case, the cilia 

 being protoplasmic organs are able to transmit stimuli, and in the case of 

 Chlamydomonas with considerable rapidity. Similarly, the latent period of 

 induction and the duration of the after-effect are exceedingly short in 

 rapidly motile zoospores. It is worthy of note that zoospores, even when 

 radial, are capable of phototactic, geotactic, and chemotactic reactions, 

 although, as the result of their continued rotation, they are in a similar 

 condition to a plant rotated on a klinostat. Hence a rotating vertici-basal 

 zoospore when it reacts to light must direct one end towards the source 

 of illumination so that the axis of rotation is at right angles to the light 

 rays. It is, however, also possible that the unequal stimulation of any 

 pair of opposite sides might suffice to produce a tactic response, although 

 none would be possible if the axis of rotation was at right angles to 

 the direction of the orienting agency and both ends of the organisms were 



1 Pfeffer, 1. c., p. 463 ; 1888, Bd. n, p. 631. 



8 Verworn, Bewegung d. lebendigen Substanz, 1892^.44; Rhumbler, Ergebnisse d. Anatomic 

 u. Entwickelungsgeschichte, 1899, Bd. viu, p. 584. 



