354 LUMINOSITY IN PLANTS. 



food, but rather perforins an osmotic function, by rendering the 

 culture medium more or less isosmotic to the cell contents jof the 

 bacteria. Other salts can in the same way replace table salt, as 

 potassium chloride, magnesium chloride, calcium chloride, potassium 

 nitrate, potassium iodide, and potassium sulphate. In fact, I ha\e 

 tlie impression that potassium nitrate is more active in causing 

 luminosity than the chlorides, such as sodium and potassium 

 chlorides. 



We are indebted to Beijerinck for some exhaustive and valuable 

 investigations upon the relations existing between nutriment, lumi- 

 nosity, and growth. The method of his investigations is essentially 

 the spreading upon thin glass plates of gelatin containing photo- 

 bacteria and supplied with an excess of nutriment. When it is spread 

 out as a thin film the bacterial field quickly becomes luminous. As 

 soon, however, as the excess of nutriment is consumed, the light 

 ceases. If now we add to the gelatin a substance the influence of 

 which on luminosity and growth we desire to test, it dissolves and is 

 disseminated in a circle in all directions. If this added substance 

 is a nutritive one for luminosity, we see, frequently in a few seconds, 

 the area that was affected growing luminous. By this method 

 bacterial fields exhibit reactions of astounding delicacy. Certain 

 materials, preeminently lebulose and glucose, cause the field to grow 

 luminous in a few seconds. In this respect the photobacteria react 

 with so minute a quantity of material that Beijerinck saw in these 

 reactions an analogy to the Bunsen-flame reaction. In one sense 

 this bacterial reaction is superior, in that it continues longer. 



The luminous bacteria act in various ways with materials contain- 

 ing carbon and nitrogen. One class, called by Beijerinck Pepton- 

 bacteria, finds the necessities for growth and light development sup- 

 plied in pepton or some albuminous material; the other class, called 

 by him Pepton-carbon-bacteria, requires at the same time the pres- 

 ence of material containing pepton to supply the necessary oxygen 

 and also carbonaceous matter, which is not necessarily free from 

 nitrogen. 



If the nutritive material is well adapted to both growth and a 

 multiplication of bacteria, it will cause not only luminous fields, but 

 fields of growth called " auxanogrammes," characterized by the in- 

 numerable colonies of bacteria that develop far more rapidly in 

 the field where the material has been diffused than outside of it. 

 Beijerinck calls such nutritive material " plastic." Luminous sub- 

 stances are uniformly plastic though the reverse of this is not neces- 

 sarily true. From this the important fact follows that light devel- 

 opment by the luminous bacteria is not necessarily connected with 

 either growth or respiration. 



