CHAP. TI.] CHEMISTRY, ETC., OF PLANT LIFE. 65 



Having shown the importance of light in connection 

 with the very existence of plant life, it is necessary to 

 enter a little more in detail respecting the influence 

 exercised by this agent. It is generally known that 

 what is termed solar light or popularly speaking sun- 

 light, although apparently a white or colourless light, 

 consists in reality of a mixture of several rays of light of 

 different colours. These colours can be separated from 

 each other by proper means, and constitute the solar 

 spectrum, the colours being arranged in the following 

 order : red, orange, yellow, green, blue, indigo, violet. 

 Each colour possesses an individuality of its own, and 

 so far as plant life is concerned it has been proved by 

 experiment that the red end of the spectrum that is, 

 from the red to the green ray inclusive is alone of value 

 in enabling plants to effect certain chemical changes. 

 As an example : if a plant showing the yellow colour due 

 to being grown in darkness is exposed to the rays of 

 the red end of the spectrum, the bright green chlorophyll 

 will soon become visible, whereas if exposed to the violet 

 end of the spectrum blue to violet no change in colour 

 or formation of chlorophyll takes place. In like manner 

 assimilation, or the formation of starch within the chloro- 

 phyll-grains depends entirely on the rays of the red end 

 of the spectrum, no starch being formed when exposed 

 to the violet end of the spectrum. The yellow ray is 

 most powerful in promoting the formation of both chloro- 

 phyll and starch. 



On the other hand all physical work promoted by 

 light, as mechanical movements, are entirely due to 

 the rays of the violet end of the spectrum. If seed- 

 lings of any twining plant be taken and exposed to 



