LUMINOSITY IN PLANTS. 355 



Beijeriiick has with <>r(>at ability made use of the himinoiis bac- 

 teria for detecting the most minute quantity of an enzyme." 



The following example wnll illustrate this: He takes advantage 

 of the fact that Photohacterium j^ho.sphorescens displays light with 

 maltose, while Photohacterium Pfiujeri does not. He uses a thor- 

 oughly cooked mixture of sea water containing 8 per cent of gelatin, 

 1 per cent of pei)ton, and one-fourth per cent of potato starch. To 

 a portion of this he adds an excess of PJiotohncter'nim pho.spho?'escens 

 and to the rest the same of Pliotohacteriiim PfiUjeri^ and prepares 

 from these two similar gelatin plates equally illuminated. Tn both 

 the starch remains unchanged, seeing that these bacteria are imable 

 to secrete the necessary enzyme, diastase. If now some diastase prep- 

 aration (such as maltose, pancreas-diastase, or ptyalin) is added to 

 these plates, it distributes itself in all directions, transforms the 

 starch into grape sugar, and upon the field of the Photohncterhim 

 'pJw-splioreseenH there instantly appear strong, shining flecks, which 

 later sj^read over the whole field of growth, Avhile on the field of 

 Photohacterium P-fl/iigeri nothing of this kind is to be seen. In this 

 ^'ay Photohacteriam phospho?'escens can be made to demonstrate 

 through its luminosity the presence of maltose — that is to say, of 

 a diastase. 



For an understanding of the nature of light development in plants 

 it is above all necessary to state that the luminosity is absolutely de- 

 pendent upon free oxygen. The light is conditioned on oxidation. 

 The finest investigations of the dependence of luminosity upon oxy- 

 gen are the l^rilliant experiments of Beijerinck. According to his 

 researches the luminous bacteria ati'ord the most sensitive tests for 

 oxygen that Ave possess. Thus the extremely minute quantity of oxy- 

 , gen which unicellular alga^ give oil' in sunlight by their assimilation 

 of carbonic-acid gas, is sufficient to instantly render these bacteria 

 luminous. If we introduci' these green cells into a glass tube filled 

 with bouillon containing luminous bacteria, the light quickly disap- 

 pears l)ecause the bacteria speedily consumes the oxygen contained in 

 the licjuid. If, now, such a tube is kept in a dark room and then the 

 light of a match is allowed to fall upon it for a single second the en- 

 tire mass grows luminous. The green cells give off oxygen, and this 

 fabulously minute quantity of the free gas is sufficient to cause lumi- 

 nosity in the bacteria. It is a remarkable example of the fact that 

 physiological methods not (>nly compete well with the best physical 

 and chemical methods, but plainly surpass them, and that a vital 



a An enzyme is a product of certain plant or animal cells by means of which 

 food material of a certain l<ind, such as starch, is transformed into another 

 food material, such as grape sugar. Diastase is an example of an enzyme. — 

 7'ranslator. 



