66 BRITISH PLANTS 



Light. 



An ordinary beam of white light is a bundle of many- 

 coloured rays ; mixed together and falling all at the same 

 moment upon the retina, they produce upon the brain an 

 effect which is manifested in the consciousness as white 

 light. A beam of light may be resolved into its con- 

 stituent coloured rays by passing it through a triangular 

 prism of glass. Not only are the rays bent or refracted 

 in passing through the prism, but each coloured ray 

 is bent to a different extent, so that, if the emerging 

 beam is caught on a screen, a multicoloured band of light 

 is produced which is known as the solar spectrum. This 

 spectrum contains all the colours of the rainbow, one 

 colour fading into the next imperceptibly. Nevertheless, 

 seven primary colours may be recognized following each 

 other in order : red, orange, yellow, green, blue, indigo, and 

 violet. The red rays are the least refracted by the prism, 

 the violet rays the most. This luminous spectrum, how- 

 ever, does not contain all the rays which pass through the 

 prism. The band is continued at either end by rays 

 which, though invisible, produce other effects. The dark 

 rays beyond the red end of the visible spectrum, falling 

 upon a body, raise its temperature (dark heat-rays) ; those 

 beyond the violet end do not perceptibly warm, but they 

 act powerfully upon a photographic plate, and so are 

 chemically very active (ultra-violet or actinic rays). 



Now, these variously coloured rays are of different 

 value in their influence upon plants. The manufacture 

 of starch in the green leaf is possible only in the presence 

 of light, and it is easy to find out which rays are the most 

 useful by growing plants under coloured screens. Under 

 a red or yellow screen a plant will be found to be as active 

 in making starch as if it were living in ordinary white 

 light ; in a blue or violet light little or no starch will be 

 formed. We conclude from this that the red and yellow 

 rays of the solar spectrum are the most effective in 

 photosynthesis, and that the green chlorophyll present 

 in the assimilating cells absorbs these colours from the 

 beams, and utilizes them in the construction of carbo- 

 hydrates. This selection of rays accounts also for the 

 colour of green plants ; the green pigment absorbs the 

 red and yellow rays, but rejects the green ; the rejected 



