REPETITION OF STIMULI 1 7 



will be transmitted through the section of the stem which has been 

 killed. After this has been demonstrated the treated portion 

 should be examined by cutting sections and placing under the 

 microscope. Impulses are thus seen to traverse dead tissues, and 

 have been proven to pass through even desiccated portions. 



24. Repetition of Stimuli. A single stimulus to produce a 

 reaction must have a certain intensity determined by the organ- 

 ism, and any force when applied in a less degree does not call out 

 the full response. The stimulating force does cause disturbances 

 in the molecular motion of the protoplasm however, even when 

 too weakly applied to produce a reaction. This disturbance en- 

 dures a brief time and unless supplemented, its excitatory effect 

 is lost. If however, one insufficient application of the force is 

 followed by a second, or by a series before the influence of the 

 preceding has been lost, the effects of the successive applications 

 of the force may be added to each other and finally accomplish 

 excitation. In this way a series of weak applications of a force 

 may produce stimulation. If the strength of each of the series of 

 applications is the to increased point where each alone would con- 

 stitute a stimulus, the reaction resulting will be of greater ampli- 

 tude than that resulting from a single stimulation. The continu- 

 ation of the series of stimuli after the reaction has been shown 

 will have the effect of holding the organ or organism affected in 

 a contracted or reacted state known as tetanus. The tetanized 

 condition is accompanied by an increased release of energy on 

 the part of the organism, as if it were undergoing a number of 

 successive reactions. After a time, however, which varies with 

 the character of the stimulus and the organism, the release of 

 energy undergoes a diminution and if the stimulus is not so strong 

 as to throw the living matter into a state of rigor, the organism 

 becomes accustomed to it, and even resumes its normal condition 

 during the continuation of the stimulating force. This accommo- 

 dation is most marked and necessary in the relations of the plant 

 to radiant forces but it is also shown toward others, especially 

 that of chemical action. 



