Have Seeds, Will Travel
OSGF
Imagine for a moment that you are a plant. Your leaves are green thanks to chlorophyll, you have stems or branches to give you height, and flowers to attract pollinators to you. You are also, more or less, bound to the same spot you were planted or germinated from. Unlike us humans, plants can’t simply walk away and relocate to an area that’s more to their liking— they’re stuck. Thankfully, their not cemented entirely as the development and subsequent production of seed is one of the central ways a plant is able to reproduce, move their genetic line or have any hope of persisting beyond its life cycle. It’s for these reasons that plants have developed unique and ever-evolving modes for dispersal of their seed.
The earliest seed-bearing plants (Spermatophytes) first emerged during the Devonian period, about 319 million years ago, and began to flourish with the development of vascular plants in the Early Cretaceous period. The oldest genus of seed plants, Spermopteris, was discovered in the Lawrence Shale Formation. Given the benefits of protection, nourishment and the allowance of plants to disperse the next generation, seed bearing plants became the most successful of these early evolutionary organisms.
More broadly, plants were also evolving in tandem with surrounding ecological factors. These biological and environmental factors simultaneously shaped responses in seed bearing species. As a result, they developed morphological adaptations to aid in success for their offspring, including different modalities of dispersal:
Anemochory: dispersal by wind
Bolochory: dispersal by explosion
Hydrochory: dispersal by water
Zoochory: dispersal by animals
See below to learn about a few specific examples of the unique ways plants spread their seeds:
Dispersal by Wind:
Asclepias is easily recognizable in the native plant world. The bright orange flowers of butterfly milkweed (Asclepias tuberosa) have a strong association with the monarch butterfly, as it is one of the primary host plants for monarch caterpillars. Because of this connection and the overall benefits native milkweed provides, has become a strong staple in pollinator gardens. The habitats of Asclepias are predominantly woodland margins, roadsides, pastures and other open grasslands.
Butterfly milkweed (Asclepias tuberosa) dehisced seed follicles.
Once flowering is finished the brown and beige capsules start to develop and open. Holding hundreds of small silky filaments, the milkweed seeds wait for a strong breeze to come. When it does, the light and feathery filaments allow them to be lifted up into the air and carried to a nearby area. Sometimes, this means only a short journey of a few feet, other times they can be dispersed much longer distances.
Not to be outdone, maples (Acer sp.), cattails (Typha sp.) and many others exhibit similar kinds of aeronautics. If you are interested in the artistry and allure milkweeds offer in the winter landscape, join us for our one day workshop with Virginia botanical artist Lara Call Gastinger.
Dispersal by Explosion:
Flowers of witch-hazel in OSGF Wildlife Garden.
Our native witch-hazel (Hamamelis virginiana var. virginiana) is currently flowering in the Rokeby Wildlife Garden, sporting cheery yellow flowers. The common name may allude in part to this very peculiarity as people who witnessed it in flower after leaf drop believed there was no other way this was possible other than for it to be bewitched. Another reason might be that it was also used as a water witch to locate underground water sources. The bark also has been utilized for the astringent properties and is still used today in soaps, lotions, and other cosmetic products.
Witch-hazel seeds, photo via Capital Naturalist.
In its broad native habitat, this small understory tree or shrub is subject to competition with other understory species vying for the same resources. Hamamelis however have been able to circumvent this through the development of ‘catapulting’ seeds, which project out from the plant’s immediate surroundings up to 30 feet! More specifically, at a critical point in the summer months of June and July, the endocarp (the inner layer of a fruit) is pinched and the built up stores of pressure are released. This sends the sleek black seeds out into the air and further away from the parent plant where, theoretically, they have a better chance of surviving than relying on gravity alone.
Restriction of water resources have led many desert plants to exhibit this projectile means of seed dispersal and a slew of other cultivated plants.
Dispersal by Water:
When we think of water and its association with plants, it’s often in the context of the biological factors they need for survival. With the surface of our planet covered over two thirds with water, it only makes sense that seeds would also utilize this to their advantage. In a subdivision of hydrochory, nautochory refers to seeds floating on the surface of the water, being distributed to other areas by the water's current.
Bald-cypress (Taxodium distichum) has been around in North America since the Upper Cretaceous period. Their native habitat is primarily swamps, lake margins, and river banks, with a significant population on the Black River in North Carolina dating over 1500 years old. The “knees” that sometimes appear at the base and surrounding area of the tree is one response developed out of the often flooded conditions the trees are subjected to. Another is the floating seeds they produce. Taxodium are monoecious, meaning they are trees which either produce cones or seeds, but not both on the same tree. Seed producing trees will develop around the winter months of October through December at three to five year intervals. Once dropped, the seeds float and can be carried to another location until high waters recede.
This buoyant mode of transportation is also displayed in tropical species such as coconuts (Cocos nucifera), sugar apple (Annona squamosa), and sea mango (Cerbera manghas).
Dispersal by Animals:
A true spring ephemeral, bloodroot has a minimal window for myrmecochory. Photo courtesy of Michael Gaige.
Fruit may seem like the obvious example – these delicious food sources developed to attract a variety of critters. They in turn consume and excrete the seeds which have gone through scarification in their stomachs, easing the germination process. There are other means by which animals (including insects) facilitate seed dispersal. Myrmecochory is a subdivision within zoochory and specifically relates to the dispersal of seeds by ants. Insect-driven seed dispersal is a widely distributed method and has been recorded in close to 11,000 species worldwide. Several examples are found in our native spring wildflowers like bloodroot (Sanguinaria canadensis), trillium (Trillium sp.), woods-poppy (Stylophorum diphyllum), and rue anemone (Thalictrum thalictroides) just to name a few.
Photo via Naturally Curious.
As seeds develop, they form an elaiosome (a lipid rich tissue) that may either encase the seed or attach to the side like a small appendage. Foraging worker ants in the lowest herbaceous layer are attracted to these protein and lipid rich tissues. Once fully mature, the ants collect the seeds and transport them to the nest where the elaiosome is consumed and the seed is tossed aside. Through a somewhat mutualistic relationship formed between ants and these species, the transportation of a plant’s genetic material is able to continue.
Humans also play a role in the distribution of seeds and it’s an easy feat for us to achieve. Next time you go for a walk in the woods, try to contemplate how all of the plants around you got to where they are – and then check yourself for any hitchhiking burrs that may have recruited your help!
References:
Barbour, Jill R., and Kenneth A. Brinkman. “Hamamelis L.” Woody Plant Seed Manual, USDA FS, April 2008, https://dem.ri.gov/sites/g/files/xkgbur861/files/programs/bnatres/forest/pdf/witch-hazel.pdf. Accessed November 2022.
Bolmgren, K. and D. Cowan, P. (2008), Time – size tradeoffs: a phylogenetic comparative study of flowering time, plant height and seed mass in a north-temperate flora. Oikos, 117: 424-429. https://doi.org/10.1111/j.2007.0030-1299.16142.x
Eriksson, O., Jakobsson, A. Recruitment trade-offs and the evolution of dispersal mechanisms in plants. Evolutionary Ecology 13, 411–423 (1999). https://doi.org/10.1023/A:1006729311664
Friis, E. M., K. R. Pedersen & Crane, P. R. 2011. Early Flowers and Angiosperm Evolution. Cambridge University Press. 596 pp.
Hermsen, Elizabeth J. “Fruit & Seed Dispersal.” Digital Atlas of Ancient Life, Paleontological Research Institution, 24 Aug. 2021, https://www.digitalatlasofancientlife.org/learn/embryophytes/angiosperms/dispersal/.
Martínez-Berdeja, Alejandra, et al. “DELAYED SEED DISPERSAL IN CALIFORNIA DESERTS.” Madroño, vol. 62, no. 1, 2015, pp. 21–32. JSTOR, http://www.jstor.org/stable/44577538. Accessed Nov. 2022.
Nathan, Ran, et al. “Mechanisms of long-distance dispersal of seeds by wind.” Nature, vol. 418, no. 6896, 2002, pp. 409-413. nature.com, https://skat.ihmc.us/rid=1GQDJQ1M1-1S2KLYY-FW/Nathan_etal2002Nature.pdf. Accessed November 2022.
Nilsson, Christer, et al. “The Role of Hydrochory in Structuring Riparian and Wetland Vegetation.” Biological Reviews, vol. 85, 2010, https://doi.org/10.1111/j.1469-185x.2010.00129.x.
Robledo-Arnuncio, J.J., Klein, E.K., Muller-Landau, H.C. et al. Space, time and complexity in plant dispersal ecology. Mov Ecol 2, 16 (2014). https://doi.org/10.1186/s40462-014-0016-3
Van Den Elzen, Courtney L., et al. “Oh, the Places You'll Go! Understanding the Evolutionary Interplay between Dispersal and Habitat Adaptation as a Driver of Plant Distributions.” American Journal of Botany, vol. 103, no. 12, 1 Dec. 2016, pp. 2015–2018., https://doi.org/10.3732/ajb.1600312.