EXERCISE 3 



HOW PLANTS FEED 



Statement. That part of a plant's food which is obtained from the soil must be dissolved in water 

 before it can be taken up by the plant. The process by which soil plant food in solution passes into a 



plant is called osmosis. The greater flow of liquid under osmosis is always 

 from the weaker or less dense solution to the stronger or more concen- 

 trated solution. In this process the solution diffuses through the mem- 

 brane and passes into the interior of the root hairs of the plant. It may 

 then in a similar way pass into other cells of the plant. Ordinarily the 

 plant obtains its soil food in a very weak solution, being able to live 

 where there is as little as one part of such material dissolved in ten thou- 

 sand parts of water. If the plant solution or sap is weaker than that of 

 the soil solution, as in the case of dying or ripening plants, the flow may 

 be from the plant into the soil. 



Object. To demonstrate how the soil food enters a plant. 



Materials. Glass funnel or thistle tube ; thick sirup ; a large-mouthed 

 bottle with cork to fit ; and a piece of parchment paper or animal mem- 

 brane, as a hog bladder. This may be obtained at the butcher shop at 

 any time and softened in water before using. 

 The membrane stripped from a piece of 

 bologna, or the membrane of an egg obtained 

 by dissolving the shell in strong vinegar, may 

 be used. 



Fig. 4. Diagram showing move- 

 ment of water from soil through 

 the plant 



Directions. 1. Fill a bottle three-fourths 

 full of pure water. Close the small end of a 

 thistle tube or funnel and fill it almost full of sirup. Tie a piece of mem- 

 brane securely over the mouth of the thistle tube and place it in the bottle, 

 as shown in the illustration. Remove the cork from the small end of the 

 tube and immerse the tube until the level of the two liquids is the same. 

 Note what has happened at the end of an hour. Record the results every 

 hour for six hours. Join another piece of glass by means of rubber 

 tubing to the thistle tube and see how high the water will rise. This 

 process of diffusion of liquids through a membrane as stated above is called 

 osmosis. 



2. From the center of a potato cut two small cubes of equal size and 

 weigh each. Place each piece in a dish of water, and to one dish of water 

 add a teaspoonful of salt. To each dish add a few drops of iodine solution. 

 Twelve to twenty-four hours later pour off the water, dry each piece of potato 

 on a blotting paper, and weigh. Record and explain results on the opposite 

 page. Apply the principles observed to plant feeding. 



Fig. 5. Movement of liquid 

 through membrane by osmotic 

 action 



Questions. How does the amount of liquid in the tube at the close of the exercise compare with 

 the amount placed in the tube ? How did this water get into the tube ? Has any of the sirup passed 

 from the tube into the bottle ? Taste the water to determine. Explain how the solution inside the roots 

 of a plant is kept more concentrated than the solution outside the roots. Explain why ripe grapes, 

 cherries, apples, or peaches frequently burst following a rain. Why is the green fruit not so affected ? 



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