42 



PLANTS 



of carbon dioxide and the accompanying elimination of oxygen 

 is also dependent upon the opening and closing of the stomata. 



Photosynthesis 



87. The leaves of a green plant growing under normal con- 

 ditions always contain starch when the plant has been exposed 

 to sunlight for a time. The starch disappears at night or when 

 the plant is placed in the dark. It also disappears if the plant 

 is kept in an atmosphere which contains no carbon dioxide. 

 Etiolated leaves contain no starch under any circumstances. 

 It appears from these facts that starch is formed in the leaf 



only in the presence of chlorophyll, carbon 

 dioxide and sunlight. The chemical formula 

 for starch is CeHioOs, a carbohydrate deriva- 

 tive formed by the combination of CO2 and 

 H2O, thus, 6CO2+5H2O = C6H10O5+6O2, the 

 surplus oxygen being given off by the plant. 

 It will be noted that the number of molecules 

 of oxygen given off equals the number of 

 molecules of CO2 absorbed, which means that 

 the volumes of the absorbed and eliminated 

 gases are equal. 



88. The power of forming starch from inorganic matter is a 

 property peculiar to green plants, because of their chlorophyll, 

 and gives them the distinction of being the source whence all 

 organisms derive their food, since starch is the proximate or- 

 ganic form of almost all food substances. The starch is formed 

 within, or in contact with, the chloroplasts and appears first as 

 minute granules which grow by the addition of layers to the 

 outside in such a way that the surface of the fully formed grain 

 is marked by peculiar concentric lines. 



89. We now see whence the plant derives its large amount 

 of carbon. From the formula for starch it follows that 4/9 of 



Fig. 18.— Starch 

 grains, showing con- 

 centric lines of 

 growth. 



