112 APPLIED BIOLOGY 



so much oxygen is freed from carbon dioxide that the plant 

 cannot use it all. 



Oxygen Liberated by Photosynthesis. (/)) Place some water plants, 

 such as Elodea, in a glass funnel which is then placed with tube 

 upwards in a glass battery-jar filled with water. The water must 

 be deep enough to more than cover the funnel and its tube. Fill 

 a test-tube with water, and keeping its mouth below the water in 

 the battery-jar, invert it over the end of the funnel tube. Set in sun- 

 light. Bubbles of gas (chiefly oxygen) will rise from the plants and 

 displace the water in the test-tube. The gas may be tested for 

 oxygen by stoppering the test-tube before lifting from the water, and 

 then quickly inserting a glowing taper when the stopper is removed. 



Critical study has proved that all kinds of plants, as well 

 as animals, require oxygen constantly. They may get the 

 necessary amount directly from the air, or green plants may 

 get it from carbon dioxide when the carbon of that com- 

 pound is used in starch-making during daylight. There is, 

 then, no real difference between the breathing of mushrooms 

 or animals and plants with chlorophyll. The increase in 

 oxygen and decrease of C0 2 in the air around green plants 

 during daylight is obviously due to the independent process 

 of photosynthesis, not to their breathing. 



There are good reasons for believing that roots of plants 

 absorb oxygen from the air, which is abundant in good soil. 

 In fact, one scientific reason for cultivating or tilling the soil 

 is to mix air with the soil particles. The water of the soil 

 contains oxygen in solution, just as the water in a river 

 contains oxygen which fishes can absorb by their gills. When 

 the water is taken up into the plant stem it probably carries 

 along with it some oxygen, which is absorbed by the cells 

 with which the water comes into contact. 



Also, some of the large tubes of the fibro-vascular bundles 

 are filled with air, probably taken in chiefly at the leaf- 

 pores and also at the bark-pores (lentides), which are slit- 

 like openings in the bark leading to internal air-spaces. 



