There are both male and female Jack-in-the-Pulpits, and nutrition determines which gender a given plant is. For the first year or two, every Jack-in-the-Pulpit bears male flowers. Then the amount of nutrients the plant takes up begins to influence the sex of the plant. Females flowers produce seeds, and it takes a considerable amount of nutrients to do so. Thus, if there’s an abundance of nutrients one summer, a plant is female the following summer; a lack of nutrients produces male Jack-in-the-Pulpits the following year.
While the flowers themselves are very distinct (females are green knobs, males are threadlike and not green), it can be hard to see them, as the spathe (pulpit) wraps around the spadix (Jack) which bears the flowers at its base. You can often guess the sex of a Jack-in-the-Pulpit by the number of leaves it has. In general, female plants produce two leaves, whereas male plants usually have only a single leaf. If nutrients are really lacking, the plant typically produces a single leaf, but no Jack or pulpit. (Photo: female Jack-in-the-Pulpit on the left; male Jack-in-the-Pulpit on the right).
Naturally Curious is supported by donations. If you choose to contribute, you may go to http://www.naturallycuriouswithmaryholland.wordpress.com and click on the yellow “donate” button.