CH. Ill, 3] SYNTHESIS OF FOOD 25 



round number, gives us a useful conventional expression, or 

 constant, for the process as a whole, even though it has no 

 validity as applied to any particular plant. This CONVEN- 

 TIONAL CONSTANT for photosynthesis, assuming the usual 

 conditions of light, is 1 gram of grape-sugar per square meter 

 ofjeaf area per hour. This amounts to 10 grams per average 

 working day, or 1500 grams per summer season, for that 

 area. In the process 750 cubic centimeters of carbon dioxide 

 are withdrawn from the atmosphere each hour, and the same 

 volume of pure oxygen returned thereto ; and this amounts 

 to 7.5 liters per day, and 1125 liters per season for the same 

 area. These figures are for plants out of doors in summer ; 

 for greenhouse plants in winter they approximate to half 

 this amount. It will interest the student to convert these 

 quantities into the more familiar terms of square yards, 

 ounces, and quarts; and it will prove better yet if he see 

 them all actually reproduced before him. Further, for the 

 sake of those to whom statistics appeal, more figures may 

 be added. In a season an average leaf produces enough 

 grape sugar to cover itself with a solid crystalline layer a 

 millimeter thick, which is 40 times thicker than the chloren- 

 chyma which makes it ; and in the process it absorbs enough 

 carbon dioxide and releases enough oxygen to form a column 

 of the same area as the leaf 1.125 meters high; and this is 

 all of the carbon dioxide in a column of air 3750 meters or 

 2.4 miles high. To balance the oxygen absorbed and carbon 

 dioxide released in the respiration of an average man for a 

 year, there is needed 150 square meters of leaf area working 

 through the summer; or in other words, to balance his 

 respiration for a year a man needs all of the oxygen which 

 would be released in a summer by the walls of a cubical 

 room of leaf surface 5 meters on an edge. 

 ^We have still to explain why both light_and chlorophyll 

 Ire essential to the r^otos^niMtrc^ormationofgrape sugar. 

 Before the elements contained in the carbon dioxide and 

 water can be recombined into sugar, they must first be 



