APPENDIX 299 



18° C, and that a gelatine plate with a melting-point of 

 29° C. remained solid during 5 or 6 hours' exposure, while 

 the spores in the exposed area were killed (Ward). An 

 alternative explanation, that the death of the spores may 

 be due to changes brought about in the nutrient medium,, 

 has also been excluded by Marshall Ward, who proved that 

 no colonies develop (provided insolation has been complete) 

 if a piece of the clear letter on the agar is placed in fresh 

 nutrient material, while it is still capable of supporting a- 

 growth of the bacillus if sown with fresh spores. He also 

 exposed a plate of dry spores and one of sterile agar under 

 stencil letters side by side, and then covered them respec- 

 tively with gelatine or agar, and with fresh spores, when 

 the former showed a clear area under the letter, whereas- 

 the latter was covered by a uniform growth. The effects 

 are therefore due to the direct action of the rays of light 

 upon the micro-organisms themselves. 



It is probable that ripe spores contain a reserve store of 

 fatty material, which would partly explain their resistance 

 to staining and to the ordinary methods of sterilisation, and 

 Duclaux has shown that vegetable oils are rapidly oxidised 

 by light. From these considej:ations, together with the- 

 facts that the effect is stronger on spores than on bacteria 

 and has the character of chemical action, Ward has deduced 

 the theory that the bactericidal action of hght may be due- 

 to its destructive influence, in the presence of oxygen, on 

 fatty matters or other reserve material. 



Applications in Nature. — The above theory seems tO' 

 explain and be supported by many facts in nature. Thus 

 chromogenic bacteria often produce their pigment only in 

 the presence of light, that is, when it would be required 

 to act as a screen, and fungi which grow in open places and 

 on the upper surface of leaves are nearly always protected 

 by having a dark, or a yellow-red, or an orange colour. Ward 



