THE LEAF AND ITS FUNCTIONS 171 



all the carbon dioxide that has entered the tree during its life, for oxida- 

 tion of food and loss of the resulting carbon dioxide during the night must 

 also be taken into account. But the remaining 5,200 pounds of carbon 

 corresponds to the amount present in the carbon dioxide of 74 million 

 cubic yards of air, which would fill a cube measuring about 34 mile on a 

 side. Proof that the great bulk of the carbon dioxide comes from the 

 atmosphere and not from the soil is afforded by experiments in which 

 plants are found to grow luxuriantly under conditions such that no dis- 

 solved carbon dioxide can be obtained by the roots. 



In photosynthesis two stable compounds are broken up, and an un- 

 stable one is formed. This involves the performance of work, in which 

 the radiant kinetic energy of sunlight is transformed into -potential energy 

 stored in the form of glucose. Oxygen readily unites with glucose to 

 restore the two original stable substances, water and carbon dioxide, and 

 when this occurs, the energy that went into the making of the glucose is 

 released. In the form of kinetic (now chemical) energy it becomes avail- 

 able for the synthesis of glucose into other carbohydrates, fats, and 

 proteins and for other forms of work. 



All life is made possible by the temporary storage and controlled release 

 of energy, and the overwhelmingly predominant (though not the only) 

 device for energy capture is the sunlight-chlorophyll-glucose mechanism. 

 Most living things are either directly or indirectly dependent upon photo- 

 synthesis for their existence. The importance of the mechanism is not 

 merely that it stores energy in the form of organic substances; it also 

 provides the means for the release of this energy through oxidation. 



Maintenance of the oxygen content of the air at something like its 

 present level is essential to animal life. Man and all other animals would 

 soon cease to exist if oxygen were not continually being returned to the 

 air through photosynthesis. Oxidation is the spontaneous process, and 

 the freeing of oxygen from combination requires expenditure of energy. 

 With almost negligible exceptions, the green plants are the only present 

 source of free oxygen. The atmospheres of the other planets contain no 

 detectable amounts of this gas, and it is reasonable to conclude that until 

 green plants became established, the earth's atmosphere was also with- 

 out free oxygen. 



SOME FURTHER ASPECTS OF PLANT PHYSIOLOGY 



The relation between photosynthesis and respiration. Respiration 

 in plants and animals is identical; in both the process includes the intake 

 of oxygen and the liberation of carbon dioxide. Each cell of an animal or 

 plant body uses oxygen and produces carbon dioxide (and water) con- 

 tinuously, at a rate that varies with the activity of the cell. In the animal, 

 the nature of respiration is plainly evident, since there is no other process 



