SUN RAYS AND PLANT LIFE ' 



By Easl, S. Johnston 

 Division of Radiatioti and Organisms, Smithsonian InstitutiQn 



[With 4 plates] 



The sun is the earth's reservoir of energy. All terrestrial activity 

 is related directly or indirectly to the sun. In the third Arthur 

 lecture Dr. Abbot discussed how the sun's radiant energy warms the 

 earth,^ and showed the dependence of the weather on variations in 

 the energy of the sun itself. Here we shall consider the influence of 

 radiant energy on plant life. 



In green plants a process takes place in the presence of light that 

 has determined and continues to determine the destiny of nations and 

 the very existence of man. This process is technically known as 

 photosynthesis. By this process carbon dioxide and water are united 

 in the presence of chlorophyll, the green plant pigment, to form the 

 simple sugars. These products are elaborated into starch and other 

 carbohydrates and into proteins, organic acids, fats, and other plant 

 synthates. Many of these compounds are food, not only for the 

 gi^een plants themselves, but also for animals and nongreen plants. 

 These foods, on being assimilated, are built into new structures formed 

 in growth and the stored energy is released. 



Green plants, by this process of photosynthesis, supply the living 

 world with food. The struggle for land rich in food resources has 

 more than once influenced the destiny of ancient as well as modern 

 people. Through the centuries the availability of food has deter- 

 mined to a large extent the size of centers of population. Transpor- 

 tation, to be sure, enters as an important factor, but this has been 

 governed in general by fuel. Coal beds and oil fields are resources 

 of potential solar energy resulting directly or indirectly from photo- 

 synthesis. Here again man, in his struggle for existence, battles by 

 brute force or cunning for supremacy. Thus one is tempted to con- 

 tinue ad infinitum with examples showing the relationship of solar 



^ The fifth Arthur lecture, under the auspices of the Smithsonian Institution, Feb. 25, 

 1936. 



* Abbot, C. G., How the sun warms the earth. Smithsonian Ann. Rep. 1933, pp. 149-179, 

 6 pis., 20 figs., 1935. 



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