254 



GENERAL PHYSIOLOGY 



suddenly and only upon stimulation, and hence it may be sup- 

 posed that the light serves as a means of frightening enemies 

 (Fig. 112). 



The spontaneous emission of light is much less common. It 

 appears especially in certain putrefactive bacteria that live upon 

 decaying sea-fish and flesh (Bacterium phosphor escens), as well as in 

 mushrooms (Agaricus), and certain insects (Mater, Lampyris). 

 Numerous researches have been carried on respecting the 

 nature of the light, e.g., those of Panceri and Secchi on Salpce 

 (Pyrosoma), those of Moseley on deep-sea ccelenterates (Alcyonaria), 

 and more recently, especially those of Langley and Very ('90) upon 

 the lightning-bug (Pyrophorus noctilucus). To obtain a comparison 



of insect-light and sunlight, 

 Langley and Very super- 

 posed a spectrum of the 

 light of Pyrophorus above 

 the solar spectrum (Fig. 

 113), and thus determined 

 that with equal luminosity 

 the solar spectrum extends 

 further toward both the 

 violet and the red than 

 the light of Pyrophorus, 

 but that the latter is more 

 intense than sunlight in 

 the green. 



It is easily understood 

 that the origin of so pe- 

 culiar a phenomenon as 

 organic luminosity has 

 especially attracted the 

 attention of investigators, 

 and it is not surprising 

 that an enormous literature upon the subject has appeared. 

 Pfliiger (75, 1, 2) has collected a series of physiologically 

 interesting accounts. It appears therefrom that very different 

 views have been put forward upon the origin of the light in 

 organisms. The idea early met with great approval, especially 

 among non-specialists, that organic light depends upon the 

 presence of phosphorus, to the mild light of which it has a certain 

 external similarity. But exact investigations have shown that it 

 has nothing whatever to do with phosphorus. This follows from 

 the fact, among others, that the emission of light presupposes life 

 in the cell. It can be observed in the single cell, a free-living 

 bacterium from decaying fish, an infusorian or radiolarian from 

 sea- water, or a tissue-cell of a composite animal- or plant-body : 

 but in every case the photogenic substance is produced only in 



FIG. 112. Noctiluca miliaris, a pelagic flagellated cell 

 which becomes luminous upon stimulation. 



