PHYSIOLOGY 329 



The streaming movements of protoplasm were discovered by CORTI in 1772. 

 Favourable examples for their demonstration are the hairs of many plants, the 

 cells of the leaves of some water plants, and the long cells of the Characeae and 

 Siphoneae. 



B. The Conditions of Locomotion 



Since these movements are due to protoplasm and its organs it 

 will be readily understood that they depend on the general conditions 

 for the life of the protoplasm. 



The existence and the activity of all these movements thus depend 

 especially on a favourable temperature, and in aerobic plants on the 

 presence of free oxygen. The protoplasmic movement can, however, 

 continue for we^ks in the absence of oxygen in the case of facultative 

 anaerobes like Nitella. Certain Bacteria that are obligate anaerobes 

 lose their motility on the entrance of oxygen ; on the other hand, 

 aerobic Bacteria which have ceased to move in the absence of oxygen 

 resume their movement when a supply of this gas is available (p. 248). 



On overstepping the minimum or the maximum for these factors 

 a loss of motility or a condition of rigor results. Thus we speak of 

 cold-rigor, heat-rigor, etc. This condition can be removed by a return 

 of the favourable conditions, but if it lasts long enough will ultimately 

 lead to death. In some cases it is sufficient that these general con- 

 ditions of life should be present, but in others the movement only 

 results on the application of a special stimulus. 



Thus it is known that protoplasmic movement often only appears on wounding 

 the plant, or is increased by this. In certain Bacteria movement is started by 

 the stimulus of light or by a particular concentration of the substratum. Other 

 external influences may lead to a loss of- motility, while movement also ceases in 

 temporarily motile objects^ such as swarm-spores or spermatozoids, as the result 

 of internal causes . 



In giving a definite direction to movements of locomotion, 

 external stimuli play a very special part. In the absence of such 

 directive stimuli plasmodia move without a destination, the direction 

 of swimming or circulatory movements may frequently be reversed, 

 and only the rotation-stream is characterised by a constant direction. 



C. Tactic Movements 



The main directive stimuli are one-sided illumination, and dissolved 

 substances unequally distributed through the water. The directive 

 movements brought about by such factors are termed tactic; that 

 effected by light is phototaxis, and that by dissolved substances 

 chemotaxis. Other less widespread tactic movements will be omitted 

 here. 



The resulting movements bring the freely motile plant or the 

 motile organ of a cell either towards or away from the stimulus ; in 



