268 GENERAL SCIENCE 



USEFULNESS OF PLANTS TO MAN 



Without plant life, and its continuous consumption of 

 carbon dioxide from the air, all available carbon of the earth 

 in time would be in the atmosphere. A corresponding 

 amount of free oxygen would have been taken from the 

 atmosphere permanently to form carbon dioxide. In the 

 round or " cycle" of changes for the element carbon, plants 

 appropriate solar energy as heat and light from the sun in 

 the chemical changes that occur in the chloroplasts. This 

 energy in turn becomes available in animals through oxi- 

 dation of the carbohydrates of the digested food. In a 

 very real sense our power to do work, both physical and 

 mental (brain activity), is from liberated energy derived from 

 the sun and made available to us through the agency of 

 plants. Without plants all this round of transformation 

 of energy would be blocked. 



The yield of dry hay from meadow land may be two tons 

 per acre. The growth of corn stalks per acre when dried 

 may weigh two tons or more. A large part of the weight 

 of the stems of grass, of wheat and other grains, of corn 

 stalks, and of the woody stems of all shrubs and trees, is the 

 substance known as cellulose (C 6 Hi O 5 ). Over 40 per cent 

 of cellulose is carbon. Plants get the food material for 

 cellulose in part from the soil through their roots in the form 

 of water, but the carbon is taken in by the leaves in the 

 form of carbon dioxide gas. A yield of fifty bushels of 

 shelled corn per acre represents a weight of nearly one and 

 one-half tons of the grain. Of this nearly one-half (45 per 

 cent) is carbon. When the amount of carbon gathered in 

 from the atmosphere by the growth of vegetation during a 

 season is estimated in tons per acre, the importance of this one 

 function of the leaves and of the green parts of the stems of 

 plants becomes apparent. In the chloroplasts, under the 



