i 4 FORESTS AND MANKIND 



ward. This capacity of the root to turn vertically down is 

 caused by a small mechanism in the very tip that operates 

 somewhat as a spirit level, making the rootlet sensitive to the 

 pull of gravitation. A little after the first root emerges, our 

 seed pushes out its bud and the minute, tender leaves of the 

 embryo enlarge and unfold. At first these leaves are white or 

 yellowish like the root itself, but soon they turn green by virtue 

 of a pigment called chlorophyll, a Greek word for leaf -green. 

 Chlorophyll is in many respects like the red pigment of ani- 

 mals. It is often said to be the most important substance in the 

 world, for practically all food on earth is primarily based on 

 its presence in green leaves, and without it all plants would die. 

 Until now our seedling has been living and growing on food 

 derived from its parent, but as soon as these young leaves 

 become green a new chemical process begins, a process by 

 which water from the soil and carbon dioxide from the air are 

 decomposed and made over, producing sugar and oxygen. 

 This process we know as photosynthesis. It can go on only 

 in light, for light is its energy source, in much the same way 

 as light supplies the energy for chemical changes that take 

 place in a photographic plate, or the chemical changes that 

 make dyes fade when exposed to sunlight. This ability of the 

 plant to produce food from its surrounding elements is made 

 possible by the power of the leaves to absorb and use the sun's 

 rays. In the last analysis the sun is the source of all energy 

 and activity on earth, for without it no leaf could manufacture 

 food and all living things would perish. Even the coal that we 

 burn today is the imprisoned sunlight of bygone ages, energy 

 taken from the sun's rays in the earlier days of the world by 

 leaves of trees long dead. The sugar thus formed in the leaves 



