34 General Botany 



reduced to one twelfth. This is still several times the intensity 

 of light in an oak, maple, or spruce forest, where one finds herbs 

 on the forest floor that must be able slowly to manufacture suffi- 

 cient food with a fiftieth or a hundredth of full sunlight. 



Chlorophyll necessary for photosynthesis. By using a plant 

 with variegated leaves, the iodine test will show that the white 

 parts form no starch. Since starch is formed only in the green 

 part of the blades, it is evident that chlorophyll is necessary for 

 photosynthesis. Any green part of a plant can carry on photo- 

 synthesis, but the principal food factories are the leaves. 



Effects of temperature on photosynthesis. The effects of 

 temperature on photosynthesis may be demonstrated by taking 

 plants that have been in the dark long enough for the starch to be 

 removed from the leaves, placing them in the light under different 

 conditions, and noting the time that it takes for starch to form. 

 Such tests show that the ordinary summer temperature is most 

 favorable for photosynthesis. When the temperature falls nearly 

 to the freezing point, photosynthesis slows up and finally ceases 

 entirely; and, on the other hand, when it rises above ioo° F., 

 the process is slowed down rapidly. 



Materials and products. Experiments have shown that the 

 materials used in photosynthesis are carbon dioxide and water. 

 Carbon dioxide is a gas that makes up from three to four out of 

 every 10,000 parts of the air. Its molecule contains one atom of 

 carbon and two atoms of oxygen (CO2). Water, which the plant 

 gets from the soil, has two atoms of hydrogen and one atom of 

 oxygen in every molecule (H2O). The simple sugars made in 

 photosynthesis from the carbon dioxide and water contain these 

 same elements (Fig. 23). 



Carbohydrates include many substances commonly classified 

 as sugars, starches, and celluloses. The simple sugars, glucose 

 and fructose, have a formula CeHi-iOe- The double sugars like 

 sucrose (cane and beet sugar) and maltose (malt sugar) may be 

 built up by combining two simple sugars. 



