nutlet [ NUHT-lit ] noun: a small nut, thick-walled achene, or stone of a drupe; one-seeded portion of the fruit of some members of the Boraginaceae, Lamiaceae, or Verbenaceae
You would probably not be surprised to hear that a nutlet is a small nut, but according to some sources, it also describes the part of the fruit of certain members of the Boraginaceae (borage family), Lamiaceae (formerly Labiatae, mint or sage family), or Verbenaceae (verbena or vervain family). These sources probably rely on American botanist Arthur Johnson’s 1931 definition of a nutlet as a “small hard-shelled fruit arising from a superior ovary, including mericarps. Borages, mints, vervains.” However, more recent works contend that “mericarp is a more appropriate term for a fruitlet of the Lamiales, and other families, in which a bilobed carpel divides into two halves as it matures” (Spjut, 2003-2015) and whose fruit is considered a schizocarp.
Experts do not always agree on what to call the fruit of particular genera, so different flora may vary in their treatment. In the case of native Cephalanthus occidentalis (buttonbush), dense, spherical clusters of 100–200 flowers hang on individual stalks at branch tips and give rise to cone-shaped fruits. Flora of Virginia describes the fruits of the genra Cephalanthus as dry and indehiscent with two halves often separating along the septum and terms the fruits of the species schizocarps, but other sources, such as the Manual of the Vascular Flora of the Carolinas, call them simply nutlets. In either case, these fruits provide food to numerous birds and waterfowl during fall migration and those not eaten then persist on the shrub providing a winter food source.
The Boraginaceae
Typical of the Boraginaceae are flowers arranged in a coiling shape (usually scorpioid cymes – see cincinnus under determinate inflorescences) and fruits, which are schizocarps that split into (usually) four one-seeded nutlets. Examples are the lovely, native spring ephemeral, Mertensia virginica (Virginia bluebells) and the non-native, invasive, perennial herb Symphytum officinale (common comfrey).
The Lamiaceae
Within the Lamiaceae are many aromatic and culinary herbs, like basil, beebalm, bergamot, giant hyssop, lavender, mint, oregano, rosemary, sage, salvia, and thyme. Usually plants in this family have four-sided stems, two-lipped (bilabiate) flowers often arranged in a verticillaster, and fruits, which the Flora of Virginia calls schizocarps, consisting of four, dry, one-seeded nutlets. Examples include native perennials Monarda fistulosa (wild bergamot), whose showy, bilabiate flowers are clustered in mostly solitary, terminal verticillasters and as the petals fall away, nutlets begin to form in the calyxes that turn from green to brown as the seeds ripen and Pycnanthemum muticum (short-toothed mountain mint), whose flowers are arranged in dense terminal and axillary verticillasters. Unusual for this family are the four nutlets of native shrub Callicarpa americana (American beauty-berry), which rather than dry are found within a fleshy drupe.
The Verbenaceae
The Verbenaceae often have four-sided stems (similar to the mint family), flowers arranged in elongated spikes, and, in North America, fruits of two to four nutlets. As its name suggests, native perennial Verbena hastata (blue vervain) is a member of this family. Its blue to purple flowers are borne on candelabra-like panicles of spikes. As new blooms progress toward the top of the lengthening spikes, fruit in the form of four nutlets develop in the calyxes that persist below. Each flower of annual Lantana camara, a tropical family member, produces two nutlets but they are contained within each shiny bluish-black drupe.
References
Acevedo-Rodríguez, P. July 2020. Guide to the Genera of Lianas and Climbing Plants in the Neotropics, Verbenaceae. Smithsonian National Museum of Natural History.
Johnson AM. 1931. Taxonomy of the flowering plants. The Century Co., New York. page 79.
Radford AE, Ahles HE, Bell CR. 1983. Manual of the Vascular Flora of the Carolinas. The University of North Carolina Press. page 979.
Simpson MG. 2019. Boraginales. Plant Systematics (Third Edition). page 400. doi: 10.1016/C2015-0-04664-0.
Spjut RW. 2003–2015. A Systematic Treatment of Fruit Types. ©The World Botanical Associates
Watson L, Dallwitz MJ. 1992 onwards. Dallwitz (1980), Dallwitz, Paine and Zurcher (1993, 1995, 2000), and Watson and Dallwitz (1991). The Families of Flowering Plants: Descriptions, Illustrations, Identification, and Information Retrieval. Boraginaceae Labiatae. Verbenaceae. Version: 14th December 2000. (accessed June 28–30, 2024).
Weakley AS, Ludwig JC, Townsend JF. 2012. Flora of Virginia. Botanical Research Institute of Texas. pages 428, 661, 672–673, 680, 901, 962.