The Prevalence of Green Color in Plants 27 



(one twenty-fifth of an inch) thick. The same quantities are 

 also expressed in a graphic way in the accompanying figure 3, and 

 still more expressively, perhaps, in figure 4. 



We must now examine more closely the photosynthetic sugar 

 and starch which appear in lighted green leaves. The microscope 

 does not show much about them, for the sugar is always dis- 

 solved in the sap of the cells, and the starch, although solid, is in 

 grains too small to be seen very clearly. Their chemistr}', how- 

 ever, is well-known and important. The sugar is of more than 

 one kind, but the commonest is that known as grape sugar, 

 or dextrose, which has the chemical composition, CgHjoOg, and 

 which is intermixed with some fruit sugar or fructrose having an 

 identical formula. This formula, I need hardly say to the reader 

 of this book, means that this sugar is composed of 6 parts of 

 carbon, 12 of hydrogen and 6 of oxygen, though why this particu- 

 lar combination of these three diverse elements should give a 

 substance with the properties distinctive of grape sugar, nobod}' 

 yet knows. IMuch less abundant in leaves is cane sugar, which 

 has the composition C12H22O11. Starch has for its formula 

 (06Hio05)/i, the n meaning a multiple, though for our purposes 

 we may treat it simply as CgH^o^s- Now it is immediately 

 obvious that these three substances, so closely associated in the 

 leaves of plants, are also very closely related in their chemical 

 composition, for they differ from one another only in their relative 

 proportions of hydrogen and oxygen. Thus, — 



CeHi^Oe - H2O = CgHioOs 

 grape sugar water starch 



CisHooOu + H2O = 2 parts CgHisOe 



cane sugar water grape sugar and fruit sugar. 



CeH.oO^ + H2O = C6H12O6 



starch water grape sugar 



2 parts C6H12O6 — H2O = Ci2H220n 



grape sugar water cane sugar 



These three important substances thus differ, so far as their 



