104 



BIOLOGY FOR BEGINNERS 



To prove that the oxygen did not come from the water, another 

 check could be used, in which the apparatus was the same, but no 

 plant was present, in which case no oxygen would be produced. 



In experimental work of this kind, the check experiments show 

 almost as much as the ones which actually " work." Merely stat- 

 ing that the water plant was put under the funnel, and that oxygen 



was produced, would not 

 prove anything. It would 

 be asked " How do you 

 know that the oxygen came 

 from the plant? " and 

 " How do you know that 

 light had anything to do with the 

 process? " both of which questions 

 are answered by the " checks." 



Transpiration. To show that 

 Plants pass off Water Vapor. A 

 thrifty cutting is tightly sealed into 

 a bottle of water and placed under 

 a bell jar; another similar bell jar 

 is set alongside, containing no plant. 



Water drops will soon be seen on 

 FIG. 29. Bubbles of gas will rise . ... 



in the funnel. From Atkinson. the mslde of the J ar Wlth the P lant > 



none on the other. As the bottle 



was sealed, no water could escape, except such as passed through 

 the leaves of the plant. As the empty jar showed no water, it did 

 not merely condense from the air, hence must have been passed 

 off by the leaves. A potted plant could be used, but the pot and 

 earth surface would have to be wrapped in oiled paper or sheet 

 rubber, to prevent evaporation. 



To show which surface of a leaf gives off this water vapor, two 

 watch glasses can be fastened, one on either side of a leaf. More 

 water will be found to condense on the glass fastened to the lower 

 surface, showing that transpiration is more active here. This is 

 as one would expect, since here the stomata are more numerous. 



Cobalt paper, which turns pink when moist, can also be fastened 



