216 PLANT PHYSIOLOGY 



able to move about will finally reach the position in which 

 it is most favorably affected by all the influences to which 

 it is susceptible. Other positions might be more favorable 

 for separate influences, but, the influences continuing con- 

 stant, the position occupied may be regarded as the best. 

 If the influences remained constant we could conceive that 

 the sensitive motile organism, which had come into the 

 place and position in which the balance of stimuli was most 

 favorable, would never move again. The influences are all 

 changing, except gravity, and hence the organism is exposed 

 to changing degrees and kinds of stimulation. We may 

 imagine that the movements of freely motile organisms 

 indicate three things the rate at which the stimuli change, 

 the degree of sensitiveness, and the rate of response of the 

 organism. The stimuli may change very rapidly,. but a dull 

 or feeble organism will not move correspondingly fast. 

 Conversely, an extremely sensitive and strong organism will 

 be in rapid motion even when conditions are in the main 

 unchanged. But since conditions are not absolutely identi- 

 cal at two different points, locomotion itself introduces the 

 organism to new stimuli. 



Diatoms, desmids, Oscillatoria, swarm-spores, ordinary 

 bacteria, sulphur bacteria ( both the red and the colorless ) , 

 plasmodia, and many other motile organisms, move toward 

 light until they reach the point of optimum illumination. 

 Locomotion directed by light is called phototaxis or helio- 

 taxis. Movement toward the light is said to be positive 

 phototaxis, away from the light negative phototaxis. Most 

 organisms are both positively and negatively phototactic, 

 depending upon the degree of illumination to which they are 

 exposed. Thus the various species of red sulphur bacteria 

 now growing in my laboratory collect on the brightest 

 parts of the jars in which they are living when the jars are 

 on a table six feet from the nearest window, and on the less 

 or least brightly lighted parts when the jars are on the 

 window-sills. 



Where locomotion, or at least a change in position of the 

 whole organ or organism, is impossible, the organs of the 

 cell are, in many cases, able to move in accordance with the 



