Exercise XXI 



PLANT GROWTH AND TROPISMS 111 



millimeter, under the dissecting microscope. If 

 you leave the experiment until the following 

 morning, your results will be much plainer. 

 Calculate the percentage increase of length per 

 hour in each of the solutions. 



Next, working in groups of four or eight, test 

 the effects of light and gravity on Avena seed- 

 lings. Each group should obtain a dish contain- 

 ing young seedlings. Weed out any seedlings 

 that are not straight. Place the dish under the 

 wooden box that is provided, at the end away 

 from the aperture that holds a light filter. At 

 the various tables in the room, the boxes contain 

 different light filters, red, yellow, blue, and green. 

 Move a microscope lamp close to the aperture, 

 so that light penetrating the filter reaches the 

 plants. Irradiate the plants in this way for 90 

 minutes. Then note whether or not they have 

 bent toward the light, what proportions have 

 responded, and about what angle the tip of the 

 plant assumes with the vertical. To make this 

 measurement more quantitative, lay the plant 

 on a piece of graph paper and trace the bend. 

 You should get a sufliciently accurate measure- 

 ment of the degree of bending to compare with 

 your neighbors' results. 



By comparing your results with those obtained 

 at other tables, grade the effectiveness of the 

 different colored lights in stimulating bending. 

 Draw a graph of this effectiveness against wave- 

 length in the spectrum (representative wave- 

 lengths: violet, 410 m/x; blue, 470 mp; green, 

 520 mn; yellow, 580 m/x; orange, 600 m/x; red, 

 650 m/i). Such a graph, when corrected for the 

 energy content of the various colored lights, is 

 called an action spectrum. 



Phototropic bending in plants, like vision in 

 animals, is mediated by light-sensitive pigments. 

 This is necessarily true; for light in order to have 

 any effect, chemical or physical, must be ab- 

 sorbed; and substances that absorb visible light 

 are pigments. The effectiveness of the various 

 wavelengths of light in stimulating vision or 

 phototropism depends in the first instance on the 

 capacity of the photoreceptor pigments to ab- 

 sorb those wavelengths. Hence an action spec- 

 trum tells us not only the region of the spectrum 



most effective in stimulating the response, but 

 by the same token the region of the spectrum 

 most strongly absorbed by the photoreceptor 

 pigment. This tells us the color of the pigment, 

 and sometimes provides a clue to its chemical 

 nature. 



Of the various pigments present in Avena seed- 

 lings, the chlorophylls a and b absorb light in 

 the blue and red, and hence are green in color; 

 whereas the carotenoids, xanthophyll and caro- 

 tene, as also riboflavin, absorb light only in the 

 blue, and hence look yellow. Judging by your 

 observations, which of these pigments might 

 possibly mediate the phototropic response? 



Test tube 



Bean plant 



CO2 FIXATION AND 

 TRANSLOCATION 



In this experiment, work in pairs. Obtain a 

 bean plant and a small test tube which contains 

 1 to 2 /xgm of radioactive barium carbonate from 

 your instructor. (Be careful not to spill any of 

 the carbonate; if you do, tell your instructor so 

 that he can get rid of it.) Tape the test tube to 

 one of the sticks provided, and place the stick 

 upright in the earth surrounding your bean 

 plant, directly underneath one of the bean 

 leaves. Adjust the height of the stick so that the 

 leaf rests firmly against the mouth of the test 

 tube, as shown in the diagram. Gently push the 



