572 PRINCIPLES OF GENERAL PHYSIOLOGY 



them in sixty seconds, without eosin. A third experiment consisted in taking light 

 of 518 fifji, that is, in the position of the absorption band of eosin, and making it of 

 about the same energy as that of the ultra-violet light previously used. Alone, 

 this light had no effect, as would be expected, since it has a longer wave length 

 than that found ineffective in the first experiment. On the other hand, in the 

 presence of eosin, which has no action in itself, as the first experiment showed, the 

 bacteria were killed in seventy to ninety seconds. It appears that the action of the 

 eosin is to be compared to that of an optical sensitiser. The above results may 

 be put in a table as follows : 



Since chlorophyll is so active as an optical sensitiser, it has a very powerful 

 " photo-dynamic " action. 



The explanation of the phenomenon seems to be of the same nature as the 

 similar one in the case of the photographic plate. The dye is adsorbed on the 

 surface of the organisms and the effect is probably produced by an activation of 

 oxygen, or perhaps by a product of the oxidation of the dye, since it requires the 

 presence of oxygen for the phenomenon to occur. 



An interesting experiment by Victor Henri, etc. (1912, p. 28), with colloidal selenium 

 throws light on the question. The solution was fluorescent, but it had no effect in the dark, 

 on certain protozoa. Under the ultra-microscope, it was seen to be a suspension of very 

 minute particles. A certain number of the organisms took up these particles into vacuoles, 

 where they aggregated into small masses. On exposure to light, it was found that only 

 those organisms which had taken up the colloid were affected, thus showing the necessity of 

 close contact and indicating a photo-chemical reaction, although not of the nature of the 

 formation of a product which could act independently of the light. If this had been the case, 

 those organisms which had not taken up the colloid would have been affected by the product 

 diffusing from the others. 



EFFECT OF LIGHT ON GROWTH 



The lethal effect of light on bacteria was first described by Marshall Ward 

 (1892), who did not recognise it as an effect of the ultra-violet rays. 



There are other cases where light is known to have a retarding action on the 

 growth of plants. Fungi, for example, grow more rapidly in the night than in the 

 day time. The phenomena of heliotropic curvature, in its permanent stage, 

 are due to diminution of the rate of growth in the places exposed to light. The 

 first effect of light, however, as we have seen (page 126), is due to change of 

 turgor, so that it is interesting to examine for a moment the effects of light on 

 the permeability of the cell membrane. Trondle (1910) showed that the result 

 depends on the intensity of the light, a weak light causing diminution ; a moderate 

 one, increase ; a very strong one, diminution again. These effects on permeability 

 correspond to those on heliotropic curvature, so that the conclusion seems justified 

 that we have to deal, in the primary stage of the latter, with changes in perme- 

 ability. V. H. Blackman (1914) has investigated the action of light on the per- 

 meability of the excitable structures of the sensitive plant. The method used was 

 that of the changes in its electrical conductivity. The interesting result was 

 obtained, that during illumination an increased permeability existed, and that the 

 first effect of cutting off the light was a further increase, after which the normal 

 state returned. Whether this result has any relation to the similar one in the 

 electric response of the retina cannot as yet be stated. 



Trondle holds that the effect of light on permeability is a complex one, 

 depending on a photo-chemical change in the membrane, together with reactions 

 on the part of the cell itself. Its use may be to facilitate the escape of assimilation 

 products. 



