LIGHT AND MOVEMENT 



haphazard, are influenced by the intensity of light so that avoiding 

 reactions occur by trial -and -error with the result that a devious path 

 is taken in a general direction away from the light ; in a favourable 

 environment the animal pursues a straight course, but entering an 

 unfavourable environment it turns away. This may be accomplished 

 by creeping or oscillatory movements as in Alg* such as diatoms and 

 desmids (Pfeffer, 1904), by amoeboid movements as in slime-fungi 

 (Stahl, 1884) or the amoiba (Mast, 1911), or by free-swimming move- 

 ments by cilia as in the swarm spores of Algse and some Ciliates 

 (Oltmanns, 1922). In some Cihates, for example, the direction of 

 movement in a uniform environment changes periodically for no 

 apparent reason so that the animal does not travel long in a straight 

 line ; when exposed to illumination the rate of change of direction is 



35 



■cipf 



ca 



Fig. 13. — Negative Ki.inokinesis in Am<kha. 



The organism is moving onto an illuminated cover-glass and eventually 

 its movement is reversed (after Mast). 



Diatom 



Desmid 



increased although the speed remains constant, so that they apjDcar to 

 avoid the light and tend to aggregate in shadow (Ullyott, 1936). In 

 comparison with orthokinesis whereby aggregation is reached entirely 

 by chance, klinokinesis, although still haphazard, is obviously a more 

 effective mechanism of orientation to attain an optimum environment 

 either towards or away from the area of the highest concentration of 

 the stimulus. 



The simplest and most primitive response of this tj^e is seen in 

 the photo-negative kinesis of Amoeba i^roteus, the reactions of which 

 have received much study. ^ The reaction is extremely elementary. 

 In a uniform environment this Rhizopod periodically throws out 

 pseudopodia in an indiscriminate way and thereby effects movement. 

 If, however, it is placed on a microscope slide with an illuminated 



1 Engelmann (1879). Davenport (1897), Mast (1910-32), Mast and Pusch (1924), 

 Folger (1925-27), Luce (1926), Bovie (1926), Mast and Hulpieu (1930), and others. 



