Sex life of Arisaema triphyllum (Jack-in-the-pulpit)

Arisaema triphyllum (Jack-in-the-pulpit) is a common plant that grows in forests throughout the eastern half of the United States. Jack-in-the-pulpit is the most common of its common names but you might also see this species called Indian turnip, brown dragon, or bog onion. Although common, this extraordinary plant species has some amazing characteristics and ecology. Jack-in-the-pulpit can change its sex (sexual hermaphrodotism or dichogamy) and this is not a one time event. This species can go back and forth between being male and female.

 

Fig. 1 Arisaema triphyllum (Jack-in-the-pulpit).

Jack-in-the-pulpit is in the largely tropical Araceae or aroid family. Like many members of the aroid family, the flowers are located on a spadix (the "Jack") which is surrounded by a spathe (Figure 1). Relatives like Symplocarpus foetidus (Skunk cabbage) have "perfect" flowers, that is they have both male and female reproductive parts, while others have male and female flowers separated on the same spadix. In Jack-in-the-pulpit, however, the plants are dioecious with separate male and female flowering plants (Figure 3). Note: You can find some plants with both male and female flowers but this is more common in its close relative Arisaema dracontium (Green dragon).

 

Fig. 3 Spadix with male flowers, not much more than four anthers.

 

Pollination is mostly accomplished by attracting and tricking fungus gnats in the Superfamily Sciaroidea, mostly Mycetophilidae family. Fungus gnats are tiny little flies, usually less than 2.5 mm long. The adults have a life expectancy of less than two weeks. During this time, they don't even eat. They just need to reproduce. Fungus gnats are attracted to Jack-in-the-pulpit by the slight fungal smell coming from the spadix. Fungus gnats normally seek out fungi to lay their eggs. In this way their larvae will have a suitable food source. But this is no fungus. Doh! They have been duped!

 

Fig. 4. Male spathe with exit hole.

To further complicate the scheme, male plants of Jack-in-the-pulpit have a hole at the bottom of the spathe (Figure 4) but the female plants do not. In this way, they allow fungus gnats to pass all the way through the male inflorescence before finally exiting out the bottom covered in pollen. The flies do this over and over until they finally reach a female plant. Since there is no hole at the bottom of the spathe they are effectively stuck to perish in the bottom of the spathe but the female flowers of the Jack-in-the-pulpit have been pollinated. Fortunately each female fungus gnat can lay hundreds of eggs and a high percentage of them do find their intended target (a fungus).

Interactions between insects and plants are so interesting and complicated.