CHAPTER XII. 



HOW PLANTS GET THEIR CARBON FOOD. 



I. The Gases Concerned. 



Exercise 26. 



128. Gas given off by green plants in the sunlight. — Take some green 

 alga, like spirogyra or vaucheria, which is in a fresh condition, place one 

 lot in a beaker or tall glass vessel of water and 

 set this in the direct sunlight or in a well lighted 

 place. At the same time cover a similar vessel 

 of spirogyra with black cloth so that it will be 

 in the dark, or at least in very weak light. 



129. The gas is shown in the form of bub- 

 bles. — In a short time we that in the first 

 vessel small bubbles of gas are accumulating on 

 the surface of the threads of the spirogyra, and 

 now and then some free themselves and rise to 

 the surface of the water. Where there is quite 

 a tangle of the threads the gas is apt to become 

 caught and held back in larger bubbles, which 

 on agitation of the vessel are freed. 



Examine the vessel which was covered to 

 exclude the light, or which was placed in the 

 dark. Are bubbles of gas given off here? 

 Place the vessel in the light and note how soon 

 bubbles begin to pass off. 



Fig- 55- 



Oxygen gas given off by 



spirogyra. 



Exercise 27. 



130. Experiment with elodea. — Take one of the higher green plants, an 

 aquatic plant like elodea, callitriche, etc. Place the plant in the water with 

 the cut end of the stem uppermost, but still immersed, the plant being weighed 

 down by a glass rod or other suitable object. If we place the vessel of water 



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