EXERCISE 6 



THE AIR AS A SOURCE OF PLANT FOOD 



Statement. The dry matter, or that portion of the fresh plant remaining after the water is 

 evaporated, is largely composed of carbon. The growing plant secured the carbon from the air. 



Object. To show that plants obtain food from the air. 



Materials. Green twigs; test tube; charcoal; iron spoon; vinegar; two pots of growing plants; 

 clay soil; a shallow pan. 



Directions, (i) Take small twigs of green wood, about one fourth inch in diameter, and place 

 them in a test tube. Heat until all the smoke and gas have been driven off, not allowing them to blaze. 



Examine what remains. Compare the pieces you 

 have heated to a piece of charcoal. Apply a flame 

 to one of the pieces. Charcoal is almost pure 

 carbon and is made in a method similar to that 

 used here. 



Boil some of the twigs, which have been 

 heated, in water. Boil some in vinegar. Place 

 the charcoal sticks in an iron spoon and heat 

 until only ash is left. What has become of the 

 carbon? If carbon passed into the air, did it 

 leave as pure carbon? If not, with what did it 

 combine and what is the compound called ? De- 

 scribe fully how carbon enters the plant, how it is 

 fixed and in what common forms it is stored in 

 the plant. 



(2) Place some finely powdered clay soil in 

 a shallow pan and add water slowly. Stir con- 

 stantly, and when a thin batter has been made of 

 the soil, pour about one-half inch of it around the 

 plants in one of the pots. Treat the two pots 

 alike during the next two weeks and observe 

 results. 



Questions. Does charcoal burn as wood burns? Does it dissolve in water? in vinegar? What is 

 the source of carbon used by plants? Do the roots absorb the carbon? Why is it necessary to have 

 air in the soil? 



References. Waters, H. J. Essentials of Agriculture, pp. 24-26. Ginn and Company. Coulter, J. M. 

 Elementary Studies in Botany, pp. 268-269. D. Appleton and Company. Lyon, Fippin, Buckman. Soils 

 their Properties and Management, pp. 480-488. The Macmillan Company. Hopkins, C. G. Soil Fertility 

 and Permanent Agriculture, pp. 13-15. Ginn and Company. 



Fig. 8. Showing the proportion of carbon and ash in corn grains 



The tube at the left contains the corn which was analyzed; the 



middle tube contains the carbon which was in the corn ; the tube on 



the right contains the ash which was in the corn. 



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