96 pringsheim's researches on chlorophyll. 



in a few minutes, the cells may be killed by heat, for in 

 other cases, e.g. in Spirogyra, after ten to twenty minutes 

 exposure the same spectrum is not entirely without effect. 

 But it is also possible to regard the death here as the result 

 of photochemical action on the protoplasm before the chloro- 

 phyll colouring matter is destroyed; and this opens the ques- 

 tion_, presently to be dealt with, of the relative sensibility of 

 the several cell-elements. 



The absolute non-activity of the red rays is, therefore, not 

 maintained, but what is established is this, that the amount 

 of the oxidising 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 deter- 

 mination almost impossible. All that can be said from a 

 consideration of the light-absorption in the chlorophyll colour- 

 ing 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 with the spec- 

 trum Z, (tig. 24), produced by transmission through a 10 mm. 

 thick layer of copper chloride solution, acts very slightly; one 

 Avith spectrum ij acts more strcmgly; whilst one with 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 

 than either of the others. Blue light produced by a solution 

 of ammonia copper sulphate, although this, as was mentioned 

 before, absorbs blue rays to a great extent, and therefore in 

 a concentrated condition allows but a feeble blue light to 

 pass, acts equally strongly. If a comparison is to be made 

 between the ligbt action of blue and green rays so produced, 

 the blue-light solution must not be too dark, for this has led 

 to many errors concerning the energy of colour in assimila- 

 tion; and it must also be remembered that the light solu- 



