222 THE CONSTITUENTS OF PLANTS. IX. 



Though the presence of iron in a plant is absolutely 

 indispensable for the formation of chlorophyll, the coloured 

 substance itself is free from that element. 



Chlorophyll and its derivatives give characteristic absorption 

 spectra, the black bands formed in the continuous spectrum 

 of white light which has been filtered through a solution of 

 chlorophyll showing the particular rays which are absorbed 

 and whose energy is utilised in the assimilative processes of 

 the plant (v. next chapter). 



Many other coloured or colour-yielding substances occur in 

 plants, but their importance in reference to the life of the plant 

 in most cases is small or not understood. 



