310 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS 



attempts to pass to a more dilute zone, an accumulation will be shown, 

 however high the concentration may be. Phobic movements do not 

 necessarily only result from a passage to zones of different concentrations, 

 but may result from changes of position in regard to an orienting agency. 

 Thus, owing to the unequal distribution of irritability over the surface of 

 the organism, every displacement might exercise a shock-effect producing a 

 return to the original orientation. From this point of view the typical tropic 

 reactions of rapidly moving organisms might be regarded as phobic responses. 

 A reversal of both the tactic and phobic responses may take place with 

 increasing concentration. Thus in the latter case, beyond a certain strength 

 the phobic movement might be excited by the passage to regions of higher 

 instead of to ones of lower concentration. In both cases, therefore, the 

 organisms may collect at a definite distance from the mouth of the capillary 

 from which the concentrated exciting solution is diffusing. Zoospores 

 ciliated on one side only show this reversal of the tactic response especially 

 well, for as the result of it they pass beyond the position of equilibrium 

 and then turning round swim back again. If the base of such an organism 

 were fixed it would presumably bend to a definite position as in the case of 

 a rooted plant, and would assume a diatropic position at some intermediate 

 point between the regions of repellent and attractive concentration. Usually, 

 however, no diatropism can be detected in freely motile organisms, although, 

 according to Verworn 1 , the ciliated Infusorian Spirostomum ambiguum 

 places itself at right angles to the direction of an electrical current, while 

 Oxytrichia and other Infusoria, which creep about with their ciliated surface 

 on the substratum may be said to be diathigmotropic. Similarly, certain 

 Desmids as well as the chloroplastids of Mesocarpiis assume diaphoto- 

 tropic positions in light of moderate intensity. Diatoms, on the other hand, 

 are ortho-phototactic, although they may be made to assume plagio-photo- 

 tropic positions by inclining the glass on which they glide at an angle 

 with the light-rays. 



Diatoms and other equipolar organisms may reverse their movement 

 without turning round, and many such organisms which normally move 

 to and fro are attracted in a definite direction merely by the movement 

 to one side lasting longer than that towards the opposite one. In Amoebae 

 and in plasmodia, however, the tropic attraction is attained by the excita- 

 tion of amoeboid movement on one side. The backward shock-movement 

 does not appear to be accompanied by any reversal of the organism, even 

 when the latter is ciliated at one end only. At least no such reversal was 

 observed by Engelmann in the case of the unipolarly-ciliated Bacterium 

 photometricum 2 . If the impact against a glass plate alters the orientation 



1 Verworn, Allgem. Physiol., 1901, 3. Aufl., p. 480. 



2 Cf. Rothert, 1. c., p. 391 ; Jennings and Crosby, 1. c., p. 36. 



