154 ANIMAL LIFE 



of a waterfall on which no light shines is as full a 

 glossy green as the velvety covering on a south wall, 

 and shows us that though light and colour, darkness 

 and pallor, are causally related, yet the intensity of 

 the necessary light and the need for its continuous 

 employment and the mode of its working have to be 

 considered before we can unravel the tangled problem 

 of animal colouration. 



In a plant these factors are comparatively easy to 

 determine. The green colour we know is associated 

 with the production of the plant's food. In the leaf, 

 starch or sugar is formed as a preliminary step towards 

 building up the plant-substance by union with juices 

 that rise as sap from the roots. For the efficiency of 

 the leaf light is requisite. Cut off all light, and the 

 leaf is starved in a day, and dies. Expose the leaf to 

 a continuous direct light, and it is no less infallibly 

 killed, though replete with starch. In the moss and 

 fern tribe darkness and starvation may long be endured 

 with impunity ; in the cactus group intense light and 

 plenty are equally well borne. But, however varied 

 their powers of adaptiveness may be, plants require 

 the green colour for a definite object namely, to lift 

 the carbon dioxide of the air to a higher power, which 

 in one form we call starch, in another sugar ; that is, the 

 green colour of plants subserves nutrition, and to do 

 this light is required. 



In this process of modifying the carbonic acid of 

 the air its oxygen is set free and made available for 

 the needs of aquatic plants, so that by day starch 



