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 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) and, according to some sources, the fruit of native Cephalanthus occidentalis (buttonbush).
In the case of 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 as schizocarps but other sources, such as the Manual of the Vascular Flora of the Carolinas, call them 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 consisting of four, dry, one-seeded nutlets. For example, the showy, bilabiate flowers of native perennial Monarda fistulosa (wild bergamot) are clustered in a rounded, terminal head and as the petals fall away, nutlets begin to form in the calyxes that turn from green to brown as the seeds ripen. The flowers of native pollinator magnet Pycnanthemum muticum (short-toothed mountain mint), arranged in dense terminal and axillary verticillasters, develop into schizocarps, which each separate into four nutlets. 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.
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.
Weakley AS, Ludwig JC, Townsend JF. 2012. Flora of Virginia. Botanical Research Institute of Texas. pages 428, 661, 672–673, 680, 901, 962.