112 PLANT GROWTH AND TROPISMS 



Exercise XXI 



leaf aside, and ring the top of the test tube with 

 vaseline. Carefully introduce 2 drops of H2SO4 

 into the bottom of the test tube with the capillary 

 tube that is provided. (Your instructor will 

 demonstrate this for you.) Make sure the acid 

 reaches the bottom of the tube, and that there is 

 not an air bubble holding back the second drop 

 of acid. If there is, hit the tube sharply with a 

 snap of your finger, until the acid falls to the 

 bottom of the tube. Also, be careful not to allow 

 any acid to wet the top of the test tube. 



As soon as the acid is in the test tube, quickly 

 replace the leaf, very gently pushing it down until 

 coming in contact with the vaseline, it is sealed 

 over the mouth of the test tube. The leaf should 

 now be left undisturbed for 10 minutes, while 

 the plant is brightly illuminated with the lamp 

 that is provided. 



You should be able to see small bubbles of 

 C^*02 rising through the acid. This is formed by 

 the reaction : 



BaCi^Os + H2SO4 -^ BaS04 + H2O + C'^Oa. 



After exactly 10 minutes, cut off the leaf at the 

 base of its stem with a pair of scissors. Carefully 

 wipe off the vaseline, and lay the leaf flat within 

 a folded piece of paper that is marked with your 

 name and the letter "A." Place the paper in the 

 refrigerator. 



Now obtain another test tube with radio- 

 active carbonate, and repeat the above experi- 

 ment on another leaf, except that immediately 

 after the leaf has been placed over the test tube 

 generating C'*02, cover the whole plant with 

 the black hood that is provided. Again, expose 

 the plant for exactly 10 minutes. 



At this point, remove the black hood and as 

 quickly as possible place the test tube generating 

 C'^Oo under a third leaf. Cut off the second leaf 

 as you did the first, remove the excess vaseline, 

 lay it within a folded piece of paper marked with 

 your name and "B," and also place it in the 

 refrigerator. 



To prepare the third leaf, again brightly il- 

 luminate the plant, and leave it undisturbed for 

 25 to 30 minutes. Now remove the leaf, and 



prepare as before, marked with your name 

 and "C." 



After the third leaf has been in the refrigerator 

 for at least 10 minutes, remove all three leaves 

 from the refrigerator and take them into a room 

 which has been outfitted as a darkroom, illumi- 

 nated only with dim red light.* There take two 

 pieces of x-ray film and cut off one corner of each 

 piece to identify an end. (Try to perform all 

 manipulations with the x-ray film in the dark, or 

 as nearly so as you can manage. The film rapidly 

 fogs when exposed to light, even to red light.) 

 Place the three leaves in the order A, B, C from 

 top to bottom on one piece of film, with the cut 

 corner at the top. Cover this with the second 

 piece of film, also with the cut corner at the top, 

 so that the leaves lie between the two emulsions. 

 Sandwich the films between two pieces of card- 

 board, holding everything together with rubber 

 bands around each end. Place the sandwich in 

 a black envelope, seal with rubber bands, mark 

 with your name, and put the package into the 

 freezer of a refrigerator. 



Sometime during the next laboratory period 

 develop your film in the dark room. Again try 

 to work in as little light as possible. Remove 

 the film from the sandwich, and throw the radio- 

 active leaves in the can which is provided. Attach 

 a clothespin to one side of each piece of film and 

 immerse both films in developer for 3 minutes, 

 then rinse in water, and immerse both in fixer 

 for 5 minutes. Then wash the films in running 

 water for at least 10 minutes before looking at 

 them. 



{Caution: Photographic developer stains, and 

 fixer eats at clothing. Keep both from dripping 

 around; and be particularly careful to keep any 

 trace of fixer out of the developer.) 



Wherever a (3-particle from the radioactive 

 carbon hits the film emulsion, a silver ion is 

 reduced to metallic silver, which in the de- 

 veloper catalyses the reduction of a whole grain 

 of the emulsion, resulting in a black spot. Com- 

 pare the patterns and intensity of radioactivity 



*If a Geiger counter is available, count the radio- 

 activity incorporated into each leaf at this point and 

 record the results in your notes. 



