50 PHYSIOLOGY. 



algse and treat them as described above gas will be given off in a 

 similar manner. 



114. We may now take one of the higher green plants, an 

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

 1 the water with the cut end of the stem uppermost, 

 but still immersed, the plant being weighted down 

 by a glass rod or other suitable object. If we 

 place the vessel ot water containing these leafy 

 stems in the bright sunlight, in a short time bub- 

 bles of gas will pass off quite rapidly from the cut 

 end of the stem. If in the same vessel we 

 % place another stem, from which the leaves 

 have been cut, the number of bubbles of gas 

 T, ,,/'^/'' given off will be very few. This indicates that 



Bubbles o: oxygen gas ° 



given off from elodea in a large Dart of the gas is furnished by the 



presence of sunlight. or- o J 



(0^1=' leaves. 



115. Another vessel fitted up in the same way should be placed in the 

 dark or shaded by covering with a box or black cloth. It will be seen here, 

 as in the case of spirogyra, that very few or no bubbles of gas will be set 

 free. Sunlight here also is necessary for the rapid escape of the gas. 



116. We may easily compare the rapidity with which light of varying 

 intensity effects the setting free of this gas. After cutting the end of the stem 

 let us plunge the cut surface several times in melted paraffine. or spread 

 over the cut surface a coat of varnish. Then prick with a needle a small 

 hole through the paraffine or varnish. Immerse the plant in water and 

 place in sunlight as before. The gas now comes from the puncture through 

 the coating of the cut end, and the number of bubbles given off during a 

 given period can be ascertained by counting. If we duplicate this experi- 

 ment by placing one plant in weak light or diffused sunlight, and another m 

 the shade, we can easily compare the rapidity of the escape of the gas under 

 the different conditions, which represent varying intensities of light. We 

 see then that not only is sunlight necessary for the setting free of this gas, but 

 that in diffused light or in the shade the activity of the plant in this respect 

 is less than in direct sunlight. 



117. What this gas is. — If we take quite a quantity of the 

 plants of elodea and place them under an inverted funnel 

 which is immersed in water, the gas will be given off in quite 

 large quantities and will rise into the narrow exitot the funnel. 



