EXERCISE 19 



Problem. What do plant cells look like when examined through a microscope ? 



What to use. Compound microscope ; slide ; cover slip ; water ; onion ; elodea or nitella ; iodine 

 solution or methyl blue. 



What to do. First remove the outer portion of the onion, then carefully peel a bit of the skin 

 from the soft, fresh part. Drop into a flat dish containing tincture of iodine or methyl blue. Place 

 flat in a drop of water on a slide ; cover with cover glass. Examine under low power and under 

 high magnification. 



Next mount a small leaf of elodea or nitella (green water plants used in aquaria). Warm by hold- 

 ing slide on hand and blowing breath upon it. Examine under low and under high magnification. 



Record. Make careful outline drawings of several cells of the onion preparation and of the green 

 plant cells. 



Note the cell wall which incloses the living fluid, called protoplasm. Part of this living fluid (the nucleus) absorbs dyes 

 and becomes stained. In green plant cells tiny green bodies called plastids float in the streaming protoplasm. These green 

 plastids are called chloroplasts. Identify all the above structures and label the drawings. 



Questions. 1. By slowly moving screw of fine adjustment (see Exercise 17) can you prove that 

 a cell has a third dimension, that is, thickness as well as length and width ? 



2. What causes the green color of leaves ? Can you tell what causes the colors of flowers ? 



3. What is your definition of a plant cell ? 



4. Look in the encyclopedia and write a paragraph on (a) Robert Hooke ; (b} Schleiden and i 

 Schwann ; (c) the cell theory. 



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