28 HUMAN PHYSIOLOGY 



Photosynthesis. If the activities of living things all 

 depend upon the oxidation of organic matter, how is 

 the supply kept up? This is one of the great questions 

 which biologists have been called upon to answer and 

 they have been able to throw much light upon it. It 

 has already been hinted that the energy of the sun 

 enters into our reckoning. It is this energy which is 

 applied to the reconstruction of fuels from the simple 

 products of their decomposition. A work like this is 

 manifestly the reverse, in a chemical sense, of oxidation 

 and can be spoken of as reduction. It is, at the same 

 time a synthesis and, since it occurs under the driving 

 power of radiant energy, a photosynthesis. 



Photosynthesis is accomplished chiefly by the higher, 

 and pigmented plants. To say green plants would be 

 nearly but not wholly correct. Every green leaf upon 

 which the rays of the sun are falling is capable of making 

 starch and other energetic compounds from the carbon 

 dioxid and water vapor which are obtainable from the air. 

 Since it is the reversal of combustion or oxidation it fol- 

 lows that when it is proceeding oxygen must be set free. 

 It is interesting to recall the occasion upon which this 

 important fact was first demonstrated. Joseph Priestley, 

 an English scholar whose life story is peculiarly absorb- 

 ing, prepared oxygen gas and observed its relation to 

 combustion in 1774. He found that in a confined vol- 

 ume of air only a limited quantity of inflammable matter 

 could be burned. But he soon made the momentous 

 discovery that the power to support combustion could be 

 restored to the exhausted air in a jar by allowing a green 

 plant to grow inside it. 



It is owing to the existence of colored plants that the 

 composition of the atmosphere changes but very slightly 

 from century to century. They abstract from it the 

 carbon dioxid which has come from all the fires in the 

 world, from all animal life, and from the vegetable king- 

 dom too, for photosynthesis does not take the place of 

 respiration in plants; it is a process which accompanies 



