The Manufacture of Food 27 



Many chemical substances kept in drug stores must be pro- 

 tected from the light; otherwise they soon change their 

 composition and become different substances. 



Chlorophyll necessary for photosynthesis. By using a 

 plant with variegated leaves, the iodin test will show that 

 the white parts form no starch. Since starch is formed only 

 in the green part of the blade, it is evident that chlorophyll 

 is necessary for photosynthesis. Any green part of a plant can 

 carry on photosynthesis, but the principal food factories are 

 the leaves. 



Effects of temperature on photosynthesis. The effects of 

 temperature on photosynthesis may be demonstrated by tak- 

 ing plants that have been in the dark long enough for the 

 starch to be removed from the leaves and placing them in 

 the light, under different temperature conditions. Such tests 

 will show that the ordinary summer temperatures are most 

 favorable for photosynthesis, and that when the temperature 

 falls nearly to the freezing point photosynthesis decreases 

 rapidly or ceases entirely. 



Materials and products. Experiments have shown that 

 the materials used in photosynthesis are carbon dioxid and 

 waten Carbon dioxid is a gas that makes up about three out 

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

 atom of carbon and two atoms of oxygen (CC^) . Water, which 

 the plant gets from the soil, has two atoms of hydrogen and 

 one atom of oxygen in every molecule (H 2 O). The carbo- 

 hydrates made in photosynthesis from the carbon dioxid and 

 water contain these same elements. 1 The simple sugars, 



1 Carbohydrates include many substances commonly classified as sugars, 

 starches, and celluloses. The simple sugars, glucose and fructose, have a 

 formula CeH^Oe. The double sugars like sucrose (cane and beet sugar) and 



