TRANSMITTED EFFECT OF LIGHT 363 



and the effect is transmitted to the hypocotyl, which 

 responds by becoming curved so that the seedling bends 

 towards light. 



It is necessary here to make special reference to the 

 confusion that arises from want of precision in the use 

 of the term stimnlus, used indifferently to denote both the 

 cause and the resulting effect. An external agent, say 

 light, causes certain excitatory change in the tissue, and 

 we refer to the agent which induces it, as the siimiUiis. 

 Thus in the instance cited above, light is the stimulus^ 

 and it is the stimulus-effect that is transmitted to a 

 distance. But in physiological literature no distinction 

 is made between the stimulus and its effect, hence arises 

 frequent use of the phrase ' transmission of stimulus'. 

 It is obvious that it is not light but its effect that is 

 transmitted. 



Such want of precision in the use of the term sti- 

 mulus would not have seriously affected the truth about 

 the description of facts, had the transmitted effect been 

 only of one kind. In a nerve-and-muscle preparation, the 

 velocity of transmission of excitation is so great, that it 

 completely masks the positive impulse (assuming the exis- 

 tence of such an impulse). The effect of indirect sti- 

 mulation is, therefore, the same as that of direct stimu- 

 lation. Any indefiniteness in the use of the term stimulus' 

 for its transmitted effect does not, in animal physiology, 

 seriously militate against the observed facts. But lack of 

 precision in the employment of the term in plant physio- 

 logy leads to hopeless confusion. For owing to the semi- 

 conducting nature of vegetable tissue, the transmitted effect 

 is not of a definite sign, but may be positive or negative ; 

 in tha first case, the response is by expansion, in the 

 latter, by contraction. Thus the transmitted effect will be 

 very different in the two cases, according as the intervcx/- 

 ing tissue is a good or a bad conductor. These facts 



