78 BOTANY. 



certain of the rays of light. The absorbed light is transformed 

 into energy which assists in the chemical changes taking place 

 in the carbonic acid (when the carbon dioxide of the air meets 

 the water in the cell it forms carbonic acid) in the cell by which 

 starch is built up. By mounting leaves of some mosses, or the 

 prothallia of ferns in water, for microscopic examination, the 

 starch grains can be seen within the chlorophyll bodies. They 

 can often be seen in the chlorophyll bodies in the leaf of 

 begonias when thin sections are made for observation under the 

 microscope. 



144. Starch in other parts of plants than the leaves. — 

 While the larger part of the starch is formed in the green leaves, 

 it is often found stored in large quantities in parts of plants not 

 exposed to the light. It is formed in the leaves during the day, 

 and at night it is dissolved and transported to other parts of the 

 plant where it may be needed for the manufacture of other 

 substances used in plant growth, or it may be stored in special 

 receptacles in the form of starch grains again, as in the potato 

 tuber, the roots of the sweet potato, or in the thick leaves of 

 the onion, etc. 



Exercise 29. 



145. To test for the presence of starch in parts of the plant where it 

 is stored. — Cut a potato tuber, scrape some of the potato at the cut surface 

 into a pulp. Apply a small quantity of a solution of iodine to this pulp. 

 Describe the result. The color produced is the reaction for what substance ? 

 Where was the starch first formed in the potato plant ? How is it that later 

 it is found in the tubers which are underground stems ? What function for 

 the potato plant does this stored starch serve ? 



If it is desired the pupils may test for starch in the enlarged roots of the 

 sweet potato, the grains of corn, or in the leaves of the onion. 



Place a small quantity of corn starch (as much as will be lifted on the 

 point of a small knife blade) in a test tube. Add water to the depth of two 

 inches and warm over a flame, then cool by moving the end in cold water or 

 by holding it under the water tap. Add to the starch water a drop or two 

 of a tincture of iodine (iodine crystals dissolved in alcohol). Observe the 

 blue color. Now heat over the flame; the color disappears because the 

 warm water extracts the iodine from the starch grains. Now cool again. 

 The blue color reappears since the starch again takes up the iodine. 



