74 ELEMENTS OF GENERAL SCIENCE 



called chloroplasts. They are colored by a very important 

 green pigment called chlorophyll, somewhat as a piece of 

 cloth may be colored by a dye. The chlorophyll may be re- 

 moved from the chloroplasts, leaving the leaf white. These 

 chloroplasts are much more numerous in the cells of the 

 palisade than in those of the spongy tissue, so that the upper 

 side of a leaf is usually much darker green than the lower 

 side. It is upon these chloroplasts that the light acts in the 

 manufacture of sugar and starch. 



78. Entrance of materials into the leaf. Water, which has 

 been absorbed from the soil by the roots, passes up the stem 

 and reaches the various parts of the leaf through the leaf 

 veins. Those cells which are not in contact with the veins 

 receive water through other cells. The water supply there- 

 fore reaches every cell of the leaf. 



Carbon dioxide, as well as other components of the air, 

 passes through the stomata by diffusion. The spaces between 

 the cells of the mesophyll allow the gas to spread throughout 

 the leaf. In this way carbon dioxide and water, the raw mate- 

 rials of food manufacture, are brought together in the cells of 

 the mesophyll. 



79. Manufacture of carbohydrates. It is in the cells of the 

 mesophyll that a change occurs in the two compounds, water 

 and carbon dioxide. The change is a complicated one, but we 

 may get some conception of its main features if we say that 

 the leaf causes the carbon of the carbon dioxide to separate 

 from the oxygen and unite with the water. Since water is 

 itself made up of hydrogen and oxygen, the molecules of the 

 new compound will contain atoms of carbon, hydrogen, and 

 oxygen. These substances unite only in certain definite pro- 

 portions. Very frequently the proportion in the compound 

 formed in the leaf is that of six atoms of carbon to twelve 

 atoms of hydrogen and six of oxygen. This is sometimes 

 represented by the symbol C 6 H J2 O 6 , and it is called grape 

 sugar. Sometimes it is starch (C e H 1Q O 6 ) that is formed, and 



