HEAT AND LIGHT AS COMPANIONS 269 



plants to transform substances absorbed from the air and the 

 ground into starches and proteins. Plants through their 

 leaves take carbon dioxide from the air, and through their roots 

 plants absorb mineral matter and water from the soil. But 

 carbon dioxide, water, and mineral matter must be changed 

 before they can be used to nourish the plant and to build up 

 tissue. When light shines upon a leaf, the green coloring 

 matter, or chlorophyll, in the leaf absorbs some of the energy 

 of the light rays, and by means of it transforms the raw materials, 

 water and carbon dioxide and mineral matter, into starch and 

 protein. Without light chlorophyll is powerless to transform 

 raw materials into food substances, and is like a mill without 

 power to run it. Food making by plants ceases at night when 

 sunlight is shut off from the earth and begins again in the morn- 

 ing when the sun's beams again fall upon the earth ; in winter 

 when there are no leaves to make food, trees and shrubs are 

 inactive and live upon food made the preceding summer and 

 stored away in stems and roots. Both chlorophyll and sun- 

 light are necessary to food making and plants like bacteria and 

 molds, which do not contain chlorophyll, cannot make food. 

 Such plants steal their nourishment, and thus live and thrive: 

 the mold which grows on jelly lives at the expense of the jelly ; 

 the bacteria which cause diphtheria live at the expense of 

 throat tissue. 



Heat and light as companions. Bodies which glow and give 

 out light are hot; the stove which glows with a warm red is 

 hot and fiery ; smoldering wood is black and lifeless ; glowing 

 coals are far hotter than black ones. The stained glass window 

 softens and mellows the bright light of the sun, but it also shuts 

 out some of the warmth of the sun's rays; the shady side of the 

 street spares our eyes the intense glare of the sun, but may chill 

 us by the absence of heat ; the setting sun takes away light and 

 heat and leaves the air chilly. In summer we lower the shades 



