CHAPTER VII. 



STARCH AND SUGAR FORMATION 

 1 . The Gases Concerned. 



121. Gas given off by green plants in the sunlight. Let 



us take some green alga, like spirogyra, which is in a fresh con- 

 dition, and 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. 



122. In a short time we note 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. 



If we now examine the second vessel Oxygen gas given off by spirogyra 

 we see that there are no bubbles, or only a very few of them. 

 We are led to believe then that sunlight has had something to 

 do with the setting free of this gas from the plant. 



123 We may now take another alga like vaucheria and per- 

 form the experiment in the same way, or to save time the 

 two may be set up at once. In fact if we take any of the green 



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