PHOTOSYNTHESIS AND RESPIRATION. 87 



as to produce carbohydrates, oxygen being given off in volume 

 equal to that of the carbon dioxide absorbed. 



Is it possible, by any artificial method, to cause the atoms 

 of carbon, hydrogen, and oxygen in carbon dioxide (CO 2 ) 

 and water (H 2 0) to arrange themselves so as to form an 

 organic substance ? To break up carbon dioxide into carbon 

 monoxide (CO) and oxygen requires great heat. How can 

 water be broken up (analysed) into its elements ? It is clear 

 that a large amount of energy is required to effect these two 

 processes of decomposition. But carbonic acid (carbon 

 dioxide dissolved in water) can be decomposed by a com- 

 paratively weak electric current (" silent discharge "). 



Here we come to an interesting point the chief product 

 of this electric action is formaldehyde (CH 2 O), a substance 

 which in watery solution ("formalin") is used for preserving 

 and disinfecting. Now, by various simple methods (e.g. boil- 

 ing with lime), formaldehyde can be converted into a sugar, 

 its molecules being condensed or packed together to form the 

 large sugar-molecules (C 6 H 12 O 6 ) this process is sometimes 

 called polymerisation. 



In green plants, sugar usually, however, cane-sugar 

 (C 12 H 22 U ) is formed in the green leaf -cells, though starch 

 is the first visible product; the starch appears as grains 

 inside the chloroplasts, while the sugar is dissolved in the 

 cell-sap. Probably part of the sugar produced is converted 

 into starch because the excess of sugar dissolved in the cell- 

 sap would cause injury to the living substance (protoplasm) ; 

 the starch, practically insoluble in water, is evidently a 

 temporary storage substance, which is later removed after 

 being converted back into sugar. 



Formaldehyde is a poisonous substance, its solution 

 (" formalin ") being used as a germ-killer. Yet it occurs 

 in small quantities in leaves, and a green plant like Spirogyra 

 can make starch when placed in water containing no carbon 

 dioxide but containing a substance which readily breaks up 

 and yields formaldehyde. 



It is now known that (1) the amount of formaldehyde 

 produced in assimilating leaves varies in exactly the same 

 way, according to the intensity of illumination, as (2) the 

 amount of carbon dioxide used and (3) the amount of starch 

 formed, being greatest in diffuse light, least in intense light 



