Nature Journal

11. Jack- (or Jill-) in-the-Pulpit

87 ß chapter eleven Jack- (or Jill-) in-the-Pulpit This chapter comes directly from a research project conducted by my student, Carolee Franklin. While it involves another oddly flowered plant, the oddity is mainly in how the plant responds to its environment before, during, and after successful reproduction. Sex is never simple, nor without its costs. While the high costs of sexual reproduction—attracting a mate, transferring gametes, and supporting the offspring—are obvious in animal species (including humans), the same lessons can be learned from a common woodland plant, jack-in-the-pulpit. “Jacks” (Arisaema triphyllum) are perennial herbs widely distributed in temperate North America, from Nova Scotia, Ontario, and Minnesota south to Florida and Louisiana. Generally occupying moist and shaded areas, like the nutrient-rich slopes along streams, the plants produce three- to five-parted leaves from a rounded corm. The corms, when dried or boiled, served as an emergency foodstuff for Native Americans and early settlers, giving the plant the common name “Indian turnip.” In addition, 88 ß chapter eleven a salve for treating ringworm and snakebite was made by mixing the raw corm with lard. (WARNING: Raw corms contain large amounts of calcium oxalate crystals which, if ingested, cause intense pain and irritation of the throat’s mucosal membranes.) Jack-in-the-pulpit leaves and inflorescences emerge from the forest floor litter in late spring. An inflorescence consists of a slender stalk (spadix) surrounded by a green- or purple-striped sheath (spathe); the latter droops protectively over the top of the entire structure, like the canopy of an old-fashioned pulpit. Borne on the lower portion of the spadix are fifty or so highly reduced flowers—all male, all female, or (occasionally) both. Upon release, the light pink pollen collects in the bottom of the cup formed by the spathe; tiny insects called thrips (Heterothrips arisaemae) drench themselves in pollen, carrying it to a neighboring plant. The resulting red berries ripen by early autumn and are consumed by woodland creatures, such as box turtles, with the seeds disseminated in their droppings. Although posing as simple herbs, jacks actually follow a complex sexual scenario—the Size-Advantage Model—similar to some advanced animals. According to this model, an individual will “choose” to be either male or female depending on the reproductive advantage to be gained; the amount of stored nutrients is a major determining factor in this choice. Large, healthy individuals, capable of sustaining the offspring, will be female; smaller and less healthy ones, required only to produce gametes, will be male; and marginal ones will “choose” not to reproduce at all, but concentrate instead on building up reserves for the next season. In jacks, this all makes for a confusing (but predictable) situation jack- or (jill-) in-the-pulpit ß 89 known as sequential hermaphroditism. From one year to the next, an individual plant may change from asexual to male or female, from male to asexual or female, or from female to asexual or male. Size, rather than history or age, is the best predictor of sexual state, since a plant’s size is a measure of its photosynthetic health—stored in the corm as starch— during the preceding year. Robust plants can “afford” to be female and support two leaves; males have only one leaf but are larger than asexual plants. Consequently, the females of a population usually develop on the most favorable sites, with males and asexuals on the harsher edges. So the microenvironment, operating through the modification of endogenous hormone levels, must be considered as the ultimate determiner of gender in jacks. This environmental monitoring system ensures that the burden of fruit production is borne by the healthiest individuals, increasing the likelihood that genetic material will be transmitted to the next generation. Jacks-in-the-pulpit, then, are “jacks” only when living less than ideal lives. As in other species, the most vigorous members of a population— and the ones bearing most of the reproductive costs—are the “jills.” Think in terms of the costs involved with reproduction. Find a tree “in fruit” and estimate the number of fruits produced. Multiply the number of fruits by the average weight of each one and calculate the total weight of fruits produced by that tree. What tiny percentage of those fruits will likely lead to successful reproduction? [3.144.42.196] Project MUSE...