HOW LIFE CAME TO EARTH 179 



as an intermediary between the colloids of the 

 green cell and the sunlight, is itself an exceed- 

 ingly complicated colloid resembling, in many 

 properties, and related to, the colouring 

 matter of the blood of higher animals. With- 

 out this green colouring matter, called chloro- 

 phyll, the other colloids of the green cell 

 could not themselves transmute the light 

 energy into chemical energy for the main- 

 tenance of the organic world. 



When a plant is allowed to develop in 

 darkness, its leaves are colourless, or etiolated, 

 as it is termed, no pigment is developed, and 

 such a plant can build up no fresh organic 

 material, and is limited to that which it 

 already possesses. It has been allowed 

 no source of energy in the sunlight, and 

 develops no mechanism to transmute it. 

 It lives and develops for a time on the reserve 

 of organic material which it possesses, and 

 then sickens and dies. But if, before this 

 happens, and while it is still colourless, it is 

 exposed to light, then the organic colloids 

 in its parts which would naturally have 

 been green, acted upon by the sunlight undergo 

 a series of energy transformations, as a 

 result of which the body called chlorophyll 

 is developed. This begins to absorb the light 

 energy, and takes into its molecule carbon- 



