Seedlings of the parasitic plant Cuscuta pentagona (dodder) hunt by directing growth preferentially toward nearby host plants. Lacking eyes, or even a nervous system, how do they detect their victims? To investigate the possibility that the parasite detects volatile chemicals produced by host plants, Runyon et al. (2006) placed individual dodder seedlings into a vial of water at the center of a circular paper disc. A chamber containing volatile extracts from tomato (a host plant) in solvent was placed at one edge of the disc, whereas a control chamber containing only solvent was placed at the opposite end. The researchers divided the disc onto equalarea quadrats to record in which direction the seedlings grew. Of 30 dodder plants tested, 17 seedlings grew toward the volatiles, 2 grew away (toward the solvent), 7 grew toward the left side, and 4 grew toward the right side.

a. Graph the relative frequency distribution for these results. What type of graph is ideal?

b. What are the relative frequencies expected if the parasite is unable to detect the plant volatiles or any other cues present? Add these expected relative frequencies to your graph in part (a).

c. Using these data, calculate the fraction of seedlings that grow toward the volatiles. What does this fraction estimate?

d. Provide a standard error for your estimate. What does this standard error represent?

e. Calculate the range of most-plausible values for the fraction of dodder seedlings that grow toward the volatiles under these experimental conditions. Does it include or exclude the fraction expected if the parasite is unable to detect plant volatiles or other cues present?

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