follicle [ FOL-i-kuhl ] noun: a dry, dehiscent fruit from a superior, unilocular (single cavity) ovary splitting along only one suture when mature
If you grow milkweed to attract monarch butterflies to your garden, then you know that they also attract milkweed bugs. You often find these bright red and black insects massed on the milkweed “pods” trying to gain access to the seeds within. But, do you know that the fruit of the milkweed is not actually a pod but rather a follicle?
Pods are technically legumes, which are similar to follicles as both develop from a superior, unilocular ovary. However, when a follicle matures and dries, it dehisces or splits along only one suture. A ripe legume, on the other hand, dehisces along both sutures from top to bottom.
Follicles can occur as simple fruits, derived from a single ovary in a flower. For example, a Paeonia (peony) fruit has five follicles, which develop from the five carpels in its single ovary. When follicles develop from multiple ovaries in a single flower, they are considered an aggregate fruit called an etaerio of follicles. The fruit of Aquilegia (columbine) also has five follicles, but they develop from five separate ovaries within the flower so are considered an aggregate fruit. Other examples include Helleborus (hellebore) and the cone-like aggregates of Magnolia.
Plants in the Apocynaceae (dogbane family) have two ovaries: in Amsonia (blue-star) and Apocynum (dogbane), the ovaries share a single style and usually produce paired follicles whereas it is more likely for Asclepias (milkweed) to produce one follicle rather than twins because both ovaries develop only in a small percentage of the flowers.
Apocynum cannabinum published by: The Morton Arboretum. (accessed through Data Portal, https://swbiodiversity.org/seinet/taxa/index.php?taxon=3709&clid=3232 , June 11, 2021).
Armstrong WP. 2001. Fruit Identification Photos #3. Buttercup Family (Ranunculaceae). Wayne’s World: 9 May 2001. (accessed June 11, 2021).
McLaughlin SP. 1982. A Revision of the Southwestern Species of Amsonia (Apocynaceae). Annals of the Missouri Botanical Garden, 69(2): 336-350. doi: 10.2307/2398939.
Morse D. 1993. The Twinning of Follicles by Common Milkweed (Asclepias syriaca). The American Midland Naturalist, 130(1): 56-61. doi: 10.2307/2426274.
Whittemore AT. Aquilegia. Flora of North America Editorial Committee, eds. 1993+. Flora of North America North of Mexico [Online]. 21+ vols. New York and Oxford. Vol. 3. http://beta.floranorthamerica.org/Aquilegia. (accessed June 11, 2021).