108 fringsheim's keseakches on chlorophyll, 



and hence the green colour is of physiological value to 

 assimilating organs, even if it play no direct part in the 

 decomposition of the carbonic acid. The illuminating light- 

 rays exercise an immediately-observable eiFect upon the 

 colourless protoplasm of the cells, especially upon the ground 

 stibstance of the chlorophyll-corpuscles and on their enclosed 

 substances. The destructive action within cells in intense 

 light, when oxygen is present, and the immunity observed in 

 its absence, indicate that the injurious effect of ligiit is due 

 to increased combustion of the cell-elements necessary to 

 life. It is still a moot-point whether the cell-elements 

 which absorb oxygen in darkness are the only ones that 

 have their affinity for the gas increased, or whether the 

 elements which, under normal conditions of darkness, remain 

 unoxidised are not also consumed in light. It is certain, 

 however, that the amount of respiration in green cells 

 incredses pari passu with the intensity of the light, and at 

 the higher intensities may reach such a degree as to kill 

 the cell, and the light affects the contents directly and not 

 through the medium of the green colour. 



The degree of sensibility to light of the tissues of different 

 plants varies greatly, and may be ascribed to the anatomical 

 character of the contents and the dimensions of the illuminated 

 cells. The great difference between green and non-green 

 cells is specially noteworthy. The former are always more 

 scTisitive than colourless cells, and, indeed, than cells having 

 any other colour besides green. It is, for example, more 

 difficult to produce light paralysis and death in blue and 

 colourless cells of Tradescantia, or in the filaments, sporangia, 

 or oogonia of Saprolegnia^ than in the larger cells of 

 Spirogyra and Nitella, or in cells of the leaves of Milium 

 or Vallisneria. 



Naturally the cause of difference is sought for in the 

 green colouring matter, and such an explanation would be 

 quite in accord with the relation of the colour to the action 

 of light in assimilation as here demonstrated. As the seat 

 of the action of light in assimilation is usually misplaced 

 in the chlorophyll colouring matter, so likewise might the 

 destruction of the cell-contents of the green cells in intense 

 light be referred to the colouring matter as the starting ])oint 

 and, by its light absorption, agent of the decomposition, or, 

 the cell-contents might be regarded as not sensitive to light, 

 and the changes taking place in them as merely secondary 

 effects of the destruction of the colouring matter. But such 

 assumptions are improbable, and, indeed, are contra- 

 dicted by the facts, that, red light, in spite of its stronger 



