HOW PLANTS GET THEIR CARBON FOOD. 



71 



containing these leafy stems in the bright sunlight, in a short time bubbles 

 of gas will pass off quite rapidly from the cut end of the stem. 



In the stem from which the leaves have been cut are there as many bub- 

 bles ? What is the reason ? What part of the leafy shoot gives rise to the 

 greater part of the gas ? 



Demonstration 23. 



131. To determine the kind of gas given off by green plants in the sun- 

 light. — Take quite a quantity oi 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 exit of the funnel. The 

 funnel should be one with a short 

 tube, or the vessel one which is 

 quite deep so that a small test 

 tube which is filled with water 

 may in this condition be inverted 

 over the opening of the funnel 

 tube. Place in the bright sun- 

 light for several days. 



With this arrangement of the 

 experiment the gas will rise in 

 the inverted test tube, slowly 

 displace a portion of the water, 

 and become collected in a suffi- 

 cient quantity to afford us a 

 test. When a. considerable 



Fig. 5 6. 

 Bubbles of oxygen gas 

 given off from elodea in 

 presence of sunlight. (Oels ) 



Fig. 57- 

 Apparatus for col- 

 lecting quantity of 

 oxygen from elodea. 

 (Detmer.) 



quantity has accumulated in the test tube, we may close the end of the tube in 

 the water with the thumb, lift it from the water and invert. The gas will rise 

 against the thumb. A dry soft pine splinter should be then lighted, and after 

 it has burned a short time, extinguish the flame by blowing upon it, when 

 the still burning end of the splinter should be brought into the mouth of the 

 tube as the thumb is quickly moved to one side. The glowing of the splinter 

 shows that the gas is oxygen. 



182. Oxygen given off by green land plants also. — If we should extend 

 our experiments to land plants we should find that oxygen is given off by 

 them under these conditions of light. Land plants, however, will not do this 

 when they are immersed in water, but it is necessary to set up rather com- 

 plicated apparatus and to make analyses of the gases at the beginning and 

 at the close of the experiments. This has been done, however, in a suffi- 

 ciently large number of cases so that we know that all green plants in the 

 sunlight, if temperature and other conditions are favorable, give off oxygen. 



