13.55 Are insects more attracted to male plants? Some scientists wanted to determine if there are sex-related differences in the level of herbivory for the jack-in-the-pulpit, a spring-blooming perennial plant common in deciduous forests. A study was conducted in southern Maryland in forests associated with the Smithsonian Environmental Research Center (SERC).28 To determine the effects of flowering and floral characteristics on herbivory, the researchers altered the floral morphology of male and female plants. The three levels of floral characteristics were (1) the spathes were completely removed; (2) in females, a gap was created in the base of the spathe, and in males, the gap was closed; (3) plants were not altered (control). The percent of leaf area damaged by thrips (an order of insects) between early May and mid-June was recorded for each of 30 plants per combination of sex and floral characteristic. Here is a table of means and standard deviations (in parentheses):

Floral characteristic level
Sex123
Males0.11 (0.081)1.28 (0.088)1.63 (0.382)
Females0.02 (0.002)0.58 (0.321)0.20 (0.035)

  1. (a) Give the degrees of freedom for the F statistics that are used to test for sex, floral characteristic, and the interaction.

  2. (b) Describe the main effects and interaction using appropriate graphs.

  3. (c) The researchers used the natural logarithm of percent area as the response in their analysis. Using the relationship between the means and standard deviations, explain why this was done.