The Manufacture of Food 33 



from the soil are brought together in the chloroplasts and united 

 to form carbohydrates. Sugar is the first abundant product, 

 but being soluble in the water of the cell, it is quite invisible. 

 In most plants a large part of the sugar is rapidly changed to 

 starch, and as the starch is insoluble in water, it accumulates 

 temporarily in the chloroplasts in the form of Uttle grains which 

 may be readily seen with a microscope. There is a very simple 

 test for the presence of starch. A solution of iodine stains most 

 substances yellow or brown, but it colors starch blue or purple. 

 So any object that contains starch — a cell, a leaf, or a piece of 

 cloth — will be colored purple if iodine is appHed to it. 



Light and photosynthesis. If we take a leaf from a plant that 

 has been in the dark for two days, place the leaf in warm alcohol 

 to remove the chlorophyll, and then put it in a solution of iodine, 

 it is stained yellow. This proves the absence of starch. If the 

 plant is then put in the Hght for an hour, a leaf tested in the same 

 way will be colored purple, showing that starch is present. Evi- 

 dently light is necessary for photosynthesis. 



It is not surprising to find that light is so effective in building 

 up compounds in the green parts of plants, for it is a powerful 

 agent in causing chemical change. You may be familiar with its 

 use in photography.. The film and the printing paper have on 

 them a layer of gelatin containing certain chemicals. Exposure 

 to the light for even a fraction of a second effects changes in these 

 which may be seen when the film or paper is developed. Many 

 chemical substances kept in drug stores must be protected from 

 the light; otherwise they soon change their composition and 

 become different substances. 



The amount of hght required iox photosynthesis varies in dif- 

 ferent plants. Among trees, for example, the beech, sugar maple, 

 and hemlock do not require as much light as the willow, cotton- 

 wood, pine, and aspen. Usually a reduction in intensity to one 

 fifth of full sunlight does not decrease the rate of photosynthesis. 

 In some shade plants the rate does not fall off until the Hght is 



