254 . LEAVES 



energy. The need of energy for photosynthesis is easy to under- 

 stand. The combining of the elements of carbon dioxide and 

 water into sugar is preceded by a process of dissociation in which 

 carbon dioxide and probably water are in part at least separated 

 into their elements. But carbon dioxide and water are very 

 stable compounds, and to separate them into their atoms requires 

 much energy. To force the atoms of C0 2 to separate requires an 

 energy expressed by a temperature of 1300 C. It is obvious 

 that sunlight will not decompose carbon dioxide and water; for, 

 if so, these elements would be decomposed in the air. Therefore, 

 the chlorophyll must change the sun's rays into a form of energy 

 which is available for bringing about these dissociations. How- 

 ever, this energy consumed in bringing about these dissocia- 

 tions is not lost, but is stored in the sugar as latent energy to be 

 released when the sugar or the compounds formed from sugar 

 are broken into simpler compounds or into carbon dioxide and 

 water. Thus another relation of photosynthesis to respiration 

 and other oxidation processes now appears. Photosynthesis 

 stores the sun's energy in chemical compounds which, when 

 broken into simpler compounds by respiration, become a source 

 of energy for all other plant or animal activities. It is also the 

 sun's energy that is released when coal, wood, oil, and other plant 

 or animal products are burned. Thus the chloroplasts, enabled 

 by their chlorophyll to utilize the sun's energy, stand out as the 

 plant structures upon which our supply of both food and energy 

 depends. 



The utilization of only certain rays of the sun accounts for the 

 color of leaves. When chlorophyll is boiled out of leaves with 

 alcohol and the solution is viewed with a spectroscope, it is seen 

 that the red and blue rays are absorbed while most of the green 

 rays are allowed to pass through. This experiment demonstrates 

 that chlorophyll uses the red and blue rays for energy and allows 

 the green rays to escape. Thus leaves are green because from 

 them only green rays come to our eyes. 



By imagining a chloroplast as a factory, the process of 

 photosynthesis may be summarized in the following way: the 

 chlorophyll is the machinery by which sunlight is transformed 

 into energy needed for the work; carbon dioxide and water are 

 the raw materials; sugar is the product synthesized; and oxygen 

 is a by-product. The veins are the lines of transportation which 



