INTRODUCTION 29 



Light, in reference to plant nutrition and growth, 

 is of all factors the most essential. Without it 

 photo-synthesis would be impossible. But all plants 

 do not get their food in this way. For there are 

 green plants, and others that do not possess chlo- 

 rophyll, which imparts the green colour to plants. 

 Some of these last are parasites, some are sapro- 

 phytes, which derive their food from other green 

 plants. But they equally need light for growth, 

 though not for nutrition. Fungi will grow in the 

 dark, but they are not so well developed, quantita- 

 tively at least, as when they grow in the open, 

 exposed to light. In this respect all non-green 

 plants are like animals which do not make food, but 

 derive it from the plant world which alone utilises 

 solar energy for the creation of food. 



To plants light is energy. To the consciousness of 

 man light is manifested in the undulations of the 

 ether which it sets up. The rays of light are made 

 up of seven primary colours — red, orange, yellow, 

 green, blue, indigo, violet. There are dark rays 

 beyond each end of a spectrum, dark heat rays at 

 the red end, ultra-violet or actinic rays at the violet 

 end. In the process of photo-synthesis it is the red 

 and yellow rays that act upon, and are absorbed by, 

 the chloroplasts in starch-formation. 



Light intensity varies, being greater at the Equator 

 than the Poles, where the light falls obliquely. It 

 is greater at mid-day, least at sunrise and sunset. 

 The seasons similarly influence light intensity, which 



