26 Science of Plant Life 



Photosynthesis. The primary step in the making of food 

 is the building of carbohydrates through the process called 

 photosynthesis (Greek : photos, light, and synthesis, putting 

 together). In this process carbon dioxid from the air and 

 water from the soil are brought together in the chloroplasts 

 and united to form carbohydrates. Sugar is the first abun- 

 dant 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 little grains which may readily be seen with a mi- 

 croscope. There is a very simple test for the presence of 

 starch. A solution of iodin stains most substances yellow or 

 brown, but it stains starch blue or purple. So any object 

 that contains starch — a cell, a leaf, or a piece of cloth — will 

 be colored purple if iodin is applied 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 w^arm 

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

 of iodin, it is stained yellow. This proves the absence of 

 starch. If the plant is then put in the light for a few hours, 

 a leaf tested in the same way will be colored purple, showing 

 that starch is present. Evidently light is necessary for photo- 

 synthesis. 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 changes. 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 in these chemically treated surfaces changes 

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



