Geocarpy

Geocarpy is a rare form of plant reproduction that practiced by peanuts and a few other plants you may, or may not, recognize.

Peanut fruits formed underground (Pollinator) CC BY-SA 3.0

While most plants wave their flowers at pollinators and then allow their fruit to swing freely, out in clear view, geocarpic plants are far more modest.

Geocarpic plants tend to live in areas that are harsh. Seasonal fires, extreme drought, and repeated freezing and thawing (solifluction) can make plant life difficult. Because of all this uncertainty, these plants have decided that it is better to push their flowers underground to develop into fruit. The floral stem, or peduncle, does all the pushing.

Types of geocarpy

The term geocarpy refers to any plant that ripens its fruit underground. There are three forms of geocarpy: hysterocarpy, amphicarpiy, and protogeocarpy. If the ovaries are fertilized above ground and then pushed underground, it is called hysterocarpy. Peanuts are hysterocarpic. If only some of the fruits are pushed underground, it is called amphicarpic. Protogeocarpic reproduction is really wild. These plants produce their flowers underground. Think about it. How are pollinators supposed to transfer pollen to the flower if it is underground?

Protogeocarpic plants have evolved a different type of flower. The stigmas, or pollen receptors, of protogeocarpic flowers push their way an inch or so above ground. Pollinators land on these threadlike, aerial stigmas, depositing pollen, which then travels down the stigma to the flower, underground. Here, the ovary is fertilized and fruit development occurs. I’m not sure why they have those names, but I like to think of plants waving their aboveground flowers ‘hysterically’ before heading underground, to remember which one is above ground.

Geocarpy of genuflecting plant (Alex Popovkin) CC BY 2.0

South African bitter cress (Rasbak) CC BY-SA 3.0

In addition to peanuts (Arachis hypogaea), South African bitter cress (Cardamine hirsuta) and the Genuflecting plant (Spigelia genuflexa) also use this unique method of reproduction.

Do you have any geocarpic plants in your garden?