26 The Living Plant 



conditions of light; but calculations have shown that for many 

 plants collectively out of doors it approximates under average 

 summer conditions to one gram for each square meter of leaf 

 area per hour (scientifically expressed 1 gm 2 1i), or one twenty- 

 fifth of an ounce per square yard per hour, and is about half that 



FIG. 4. These cubes, which are two-fifths the original size, show the amount of solid 

 crystalline grape sugar made by a square meter (or yard) of leaf in an hour, a day, and 

 a summer. 



amount in greenhouse plants in the winter. This figure con- 

 stitutes one of those useful conventional constants which the 

 reader should store in his mind, and keep ready for use. Ex- 

 pressed in a different way, a leaf forms in a summer enough 

 photosynthetic sugar to cover itself with a solid layer a millimeter 



