President's Address. 323 



and energetically in -white light. In coloured light, on the other 

 hand, there is not only a marked retardation in rapidity of their 

 appearance, but in red the light eftect is suppressed, although in dark 

 green and blue the changes are completed in a few minutes. Tlie 

 decolorisation and death of the cells, which in white light are brought 

 about in two or three minutes, are accomplished in about five minutes 

 in green and blue light. All these researches show that in any 

 yellow, green, or blue light it is easy to decolorise and kill the cells 

 of many Ahjce, Characece, Musci, Filices, and Phanerogamcv, pro- 

 vided the colour employed is not too dark ; whilst in red light of 

 the same intensity, and after twice or four times as long exposure, 

 no changes take place, and the red rays therefore appear to be proto- 

 chemically inactive, or at least only very slightly active, on plant- 

 cells. 



That these effects are not due to the heat, considerable as it must 

 be in some coloured lights, generated at the focal point of the large 

 lens used, is evident from a comparison of the changes accompany- 

 ing death in cells from heat with those here described. The 

 destruction of the colouring matter will not be confounded in the 

 two cases, but the changes in the protoplasm, the paralysis and 

 actual death of the cells, are alike, though not identical, from both 

 causes. 



The relation of the action of light to the chemical j^rocesses in 

 plant cells is made more clear when the plants exposed to the in- 

 tense light are placed in different gases or gas mixtures. If Spiro- 

 gyra or Nitella cells are exposed to light of any colour, and the 

 ordinary atmosphere of the gas chamber is replaced by one of 

 hydrogen free from oxygen, or of a mixture of hydrogen and car- 

 bonic acid, they show after twenty minutes no change of colour or 

 their normal character, and may be kept in this condition for weeks, 

 provided other detrimental agencies are excluded. Indeed, green 

 and non-green cells which, in presence of oxygen, even in the rela- 

 tively cold green and blue light, are killed after an exposure of three 

 to five minutes, remain uninjured when oxygen is excluded in light 

 of any colour, so long as the exposure is not so protracted as to allow 

 of development of hurtful heat effects. The temiDcrature obtaining 

 in such cases in the cell-contents, in each chlorophyll-corpuscle and 

 in each plasma-molecule, high as it must be, cannot then be the 

 essential cause of the appearances produced. The substitution of 

 hydrogen for the ordinary air is accompanied by no reducing action 

 causing warming of the object. The light and heat absorption of 

 the chlorophyll-corpuscles and the other constituents of the proto- 

 plasm remain unchanged, and heat effects and conduction are essen- 

 tially alike in both atmospheres. If, therefore, the destruction of 



