THE DISPLAY OF KNERGY 369 



The typical moth is positively phototropic. 'IMiis is an advantage 

 in its natural environment because it flies at night and gets its food 

 largely from white flowers which are mon^ conspicuous at night. If, 

 however, the factor of artificial light is introduced, the moth flies to 

 its death. This is not because it "thinks" it sees a white flower, but 

 because its eyes, its central nervous system, and its wings are all 

 connected as a unit, so that the animal has to turn in flying to the 

 flame not once, but again and again. 



Jennings found Paramecium equally responsive to paper, silk, or 

 particles of carmine placed in its immediate environment, thus 

 showing a purely mechanical response. It took these foreign sub- 

 stances into its gullet and the material was passed into the body. 

 Such responses are not advantageous. On the other hand a purely 

 thigmotropic response may be advantageous to these animals. Para- 

 mecia feed on bacteria, which may form raftlike masses. As soon as 

 a Paramecium comes in contact with such a mass, its response to this 

 stimulus causes it to remain quiet, while it feeds upon the bacteria. 

 Its sensitivity to other stimuli at this time is decreased, making it 

 seem as if its attention were "fixed upon its meal." 



Nature of Responses * 



The nature of a response to a stimulus depends upon the intensity 

 and nature of the stimulus as well as upon the structure of the part 

 stimulated. The nature of this response may differ greatly. In 

 unicellular organisms the entire cell may move in response to a 

 stimulus, though sometimes there is only a turning or the movement 

 of cilia on one side. If a simple animal such as Hydra is touched, 

 withdrawal of the tentacles touched may occur, or, if the stimulus is 

 more intense, the entire body may contract. In plants, responses to 

 stimuli may result in movements caused by diff'erences in osmotic 

 pressure of the cells, or in turning movements brought about by the 

 growth or turgor of certain cells. There may be glandular responses, 

 too, such as the production of nectar in flowers, or the flow of saliva, 

 or the dry mouth of "stage fright" in man. The newt gives off slime 

 when touched, and the gland cells in the skin of a toad exude poison 

 when it is roughly handled. 



As a result of response to pressure, gas is secreted into the swim- 

 bladder of some fish. Certain areas in jellyfish or in fireflies become 

 luminous when touched, while some fishes and other animals, such as 

 squid, octopuses, tree frogs, and chameleons, respond to change? of the 



