324 President's Address. 



the cell-contents takes place in an atmosphere containing oxygen, 

 but is in abeyance when no oxygen is present, those molecular 

 changes, through which dark heat rays kill the cell at a lower tem- 

 perature, do not proceed in the protoplasm. The correct explana- 

 tion of the appearances seems to be that the light influences the 

 relation of the plarit to the oxygen of the atmosphere, and that the 

 illuminating rays increase the chemical affinity of the cell-contents 

 for oxygen. The amount of this action is dependent on the colour 

 of the illuminating rays, and increases with their refraction from the 

 red to the blue end of the spectrum. 



The red rays have been shown to have no distinct photochemical 

 effect. This is true, however, only for rays of a very definite posi- 

 tion in the spectrum, from the extreme red to those of a wave length 

 of "00061 mm. But even under these, after prolonged exposure, or 

 if the cells are very sensitive, small effects are visible, though it may 

 be doubted whether all are purely chemical and some are not 

 thermal. The absolute non-activity of the red rays is not main- 

 tained, but what is established is this, that the amount of the oxidis- 

 ing action of light on the plant cells sinks in a very striking manner 

 towards the red end of the spectrum. The increase in activity 

 towards the blue end has not as yet been quantitatively estimated, 

 and a curve, expressive of the relative amount of respiration in the 

 cells for the different rays of the spectrum, cannot therefore be 

 drawn. Indeed, the varying intensity of illumination which must 

 occur in the concentration of light upon dense masses, the great 

 difference in intensity produced by slight deviations of the object 

 from the plane of intense illumination, and the want of a standard 

 for finding the amount of action within the cells before they are 

 killed by the light, make such a quantitative determination almost 

 impossible. All that can be said from a consideration of the light 

 absorption in the chlorophyll colouring matter is, that generally a 

 rise, and a decided rise, of effect takes place in the blue. 



But careful consideration of the intensity of the active colour is 

 necessary, for it is found that in high intensities the cells are killed 

 more rapidly in yellow and green. A dark and light solution of 

 copper chloride, which differ little in breadth of their absorption 

 spectra, show a great difference in their effect upon the plant cells. 

 Thus, light produced by transmission through a 10 mm. thick layer 

 of copper chloride solution, acts very slightly ; whilst one with a 

 spectrum produced by a 5 mm. thick layer of the same solution, and 

 which has a trace of blue, causes a much stronger light effect. Blue 

 li<dit produced by a solution of ammonia copper sulj^hate, although 

 this absorbs blue rays to a great extent, and therefore in a concen- 

 trated condition allows but a feeble blue light to pass, acts equally 



