88 PHOTOSYNTHESIS AND RESPIRATION. 



or in darkness. Further, it appears that (4) the strength of 

 the electric currents in green organs also varies according to 

 the light-intensity. The optimum, in all four cases, is about 

 one-fourth of direct sunlight. 



It has therefore been suggested that the light received by 

 the green tissue is transformed into electric energy and that 

 this energy causes the production of formaldehyde from 

 carbon dioxide and water. 



The term photosynthesis is often applied to what we 

 have so far called " starch-making," because it includes a 

 building-up of simple inorganic substances into complex 

 organic substances. The term photolysis has also been 

 used, because the earlier stages involve a breaking-down of 

 carbon dioxide and water. The simpler term carbon- 

 assimilation is perhaps better than either. 



The term photosynthetic carbon-assimilation is used 

 to distinguish the normal process in green plants from the 

 chemo synthetic carbon-assimilation carried on by certain 

 Bacteria (nitro-bacteria) which use carbon dioxide but do 

 not obtain energy from light. A green plant can grow and 

 make starch when deprived of carbon dioxide, but supplied 

 with sugar or with glycerine. Light is not required for this 

 form of assimilation. 



122. Why is Light required for Photosynthesis? 



Since energy is required for the carrying on of photosynthesis 

 and light is essential for this process, it follows that light is 

 necessary because it is the source of energy. It is important, 

 therefore, to study some of the properties of light. 



In books on Physics, study carefully the following topics : the cause 

 of light ; photometry and the law of inverse squares in light intensity ; 

 the composition of white light ; refraction ; the continuous spectrum ; 

 the physical and chemical properties of the less refrangible rays (red) 

 and the more refrangible rays. 



123. Effect of Light-Intensity on Photosynthesis. 



It is easy to show that the rate at which a green leaf makes 

 starch varies according to the intensity of the light. In weak 

 light no starch is formed, and this is also the case when the 

 light is too intense, but between the two extremes the amount 

 of starch formed corresponds roughly with the light-intensity. 



