STIMULI AND THEIR ACTIONS 403 



h. The Phenomena of Depression 



While light, at least in its ordinary intensity, is, according to 

 experiments thus far, not a general excitant of living substance, 

 it is still less a depressant. The few depressing effects of light 

 that have been reported must be received as such with great 

 caution ; they have been little investigated and their suggested in- 

 terpretation is extremely doubtful. 



The phenomenon, e.g., that the growth of plants in the light is less 

 than in the dark might be regarded as a phenomenon of depression; 

 it might be imagined that light directly inhibits certain metabolic 

 processes that are necessary to growth. But the growth of plants 

 is a very complicated phenomenon, one in which many different 

 factors play a rdle, and, as Sachs ^ has already emphasised, at 

 present it is impossible to judge how far light as such has a share 

 in its occurrence. 



Another depressing effect of light might be sought in the 

 influence of the latter upon the production of light by many 

 luminou.smarine animals. For example, the statement has often been 

 made that pelagic animals, such as Ctenophora and Siphonnphora. 

 when brought from the light into the dark, do not emit light, and 

 only after they have remained in the dark for some time can they 

 be made by stimuli to do so, at first feebly and later more strongly. 

 The power of producing light in these organisms appears, there- 

 fore, to be depressed by the influence of light, and since the 

 unicellular Noctilucoi are said to behave similarly, it should not 

 be assumed that the phenomenon depends upon a secondary effect 

 of light, mediated by the sense-organs and the central nervous 

 system. But the matter is very uncertain, for, although the doubt- 

 ful phenomenon has been observed by several persons, thus far it 

 has never been investigated. 



Little more is known of the depressing effects of light, and the 

 question whether light is able to call out phenomena of depression 

 at all must remain for the present undecided. 



5. The Actions of Electrical Stimuli 



In many respects the electrical stimulus stands in peculiar con- 

 trast to other stimuli. In nature it comes into contact with living 

 organisms only in exceptional cases ; this is true also of many 

 chemical stimuli but of no others. Nevertheless, it possesses 

 many properties that make its employment upon living substance 

 especially easy and convenient. It can be graded in intensity 

 more conveniently than any other and with a fineness that answers 

 the highest requirements. Further, its employment can be limited in 

 time in any desired manner. These great advantages, which have 



' Cf. bibliography : Sachs, J., . Ueber den Einfluss, etc. 



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