The smallest seeds in the world belong to orchids, the huge flower family that includes the moth orchids sold in supermarkets. The mustard seed of the old proverb is a giant next to an orchid seed. Being that small comes at a cost: an orchid seed carries almost nothing to live on and depends on another living thing to start growing.
Just How Small Orchid Seeds Are
The shortest orchid seeds are about 0.05 mm long, near the limit of what the naked eye can see.1The New Phytologist. Tansley Review No. 110. Numerical and physical properties of orchid seeds and their biological implications The thinnest are only 0.01 mm across, because orchid seeds are shaped like tiny tubes or balloons.2The New Phytologist. Tansley Review No. 110. Numerical and physical properties of orchid seeds and their biological implications The lightest weigh about 0.31 micrograms, or about a third of a millionth of a gram.3The New Phytologist. Tansley Review No. 110. Numerical and physical properties of orchid seeds and their biological implications
To picture 0.05 mm, start with a human hair, which is roughly 0.07 to 0.1 mm wide. The smallest orchid seed is thinner than that. A grain of table salt, around 0.3 mm, is many times bigger, and a poppy seed is about 1 mm, twenty times longer. You would need a magnifying lens to see the shape of a single orchid seed.
The mustard seed gets its reputation from the proverb that calls it the smallest of seeds. Real mustard seeds are 1 to 2 mm, dozens of times longer than the smallest orchid seeds.
Orchid seeds vary a lot, though. The longest reach about 6 mm, 120 times longer than the shortest.4The New Phytologist. Tansley Review No. 110. Numerical and physical properties of orchid seeds and their biological implications So “orchid seeds are smallest” means the smallest orchids, not every orchid.
The numbers per pod are just as striking. A single orchid seed pod can hold as many as 4 million seeds.5The New Phytologist. Tansley Review No. 110. Numerical and physical properties of orchid seeds and their biological implications Other orchids make only 20 to 50 seeds per pod.6The New Phytologist. Tansley Review No. 110. Numerical and physical properties of orchid seeds and their biological implications
Ripe pods split along seams and release a tan or cream powder that drifts away with the slightest breath. It looks like fine smoke, which is why botanists call these dust seeds. Under magnification each speck looks like a tiny net bag with a dark dot inside.
What Is Inside a Dust Seed
Most seeds hold an embryo, the baby plant, plus endosperm, a food store that fuels its first growth. Orchid seeds have a small, undeveloped embryo with no surrounding food store.7PubMed Central. A novel method to produce massive seedlings via symbiotic seed germination in orchids In some dust-seeded relatives the embryo is only two cells.8Seed Science Research. ‘Dust seeds’ with undifferentiated embryos and their germination in mycoheterotrophic Monotropoideae (Ericaceae)
Split a bean and you see two thick food halves with a tiny shoot between them. Those halves let a bean sprout in plain soil with just water. An orchid seed has no equivalent store, so it cannot do this.
The simple embryo has a cause. As plants evolved less food store, the embryo stayed unformed, since the food store helps guide its development.9Perspectives in Plant Ecology, Evolution and Systematics. The evolutionary ecology of dust seeds The store normally sends signals that shape the growing embryo, and when the store shrank, those signals did too.
Much of a dust seed is air trapped inside its loose outer coat.10PubMed. Transitions between the Terrestrial and Epiphytic Habit Drove the Evolution of Seed-Aerodynamic Traits in Orchids That air space helps the seed stay aloft longer in the wind.11PubMed. Transitions between the Terrestrial and Epiphytic Habit Drove the Evolution of Seed-Aerodynamic Traits in Orchids Inside the loose coat, the baby plant is a tiny ball or oval of cells, often visible under magnification as a dark or yellowish dot. It has no root or leaves ready, just a small clump of cells.
Why Orchid Seeds Need a Fungus to Sprout
In the wild, orchid seeds sprout only with help from particular soil fungi.12PubMed. Orchids acquire fungal carbon for seed germination: pathways and players These fungi live in or on plant roots in a working partnership, rather than simply rotting or sickening the plant. In most plants the exchange runs both ways, with the fungus trading minerals and water for sugar from the plant’s leaves. In an orchid seedling the flow is lopsided at first, since the seedling has no leaves to pay with.
The partner fungus supplies the carbon the seed cannot make or store.13PubMed. Orchids acquire fungal carbon for seed germination: pathways and players The seed’s cells take sugar from living fungal threads and by digesting coils of fungus inside them.14PubMed. Orchids acquire fungal carbon for seed germination: pathways and players That sugar reaches the seedling mostly as glucose broken down from a fungal sugar.15PubMed. Orchids acquire fungal carbon for seed germination: pathways and players
The order of events goes like this. Wind drops the seed on bark or soil, and a suitable fungus must already live there. The fungus grows into the seed and is partly digested. The seed swells into a small green or white blob before any leaf appears, and only then forms a shoot and roots. In one orchid, the first leaf bud formed only after the fungus moved in.16Acta Botanica Brasilica. Characterization of seed germination and protocorm development of Cyrtopodium glutiniferum (Orchidaceae) promoted by mycorrhizal fungi Epulorhiza spp. Years can pass before the plant flowers.
Without the right fungus nearby, most seeds simply never grow. Fewer than 1% of orchid seeds become plants in natural populations.17PubMed Central. A novel method to produce massive seedlings via symbiotic seed germination in orchids
The partnership also makes orchids hard to move or rescue. Digging up a wild orchid separates it from its fungus, and planting seeds in garden soil usually fails without that fungus. This is one reason wild orchids are protected in many places.
Why Plants Make Seeds This Small
Plants that make small seeds can make many more of them, since seed size trades off against seed number.18PubMed Central. The tolerance-fecundity trade-off and the maintenance of diversity in seed size A plant has a limited amount of energy to put into reproduction, so it can spend it on a few large seeds or many tiny ones, not both. Millions of seeds per pod offset the under-1% success rate.
Dust seeds fall so slowly that even light, gusty breezes can carry them like pollen or spores.19New Phytologist. CALCULATION OF THE PRIMARY TRAJECTORIES OF DUST SEEDS, SPORES AND POLLEN IN UNSTEADY WINDS Orchids grow worldwide, a spread often credited to their dust seeds.20PubMed. Transitions between the Terrestrial and Epiphytic Habit Drove the Evolution of Seed-Aerodynamic Traits in Orchids Ground-growing orchids release seeds lower than tree-dwelling ones, and their seeds hold more air to float longer.21PubMed. Transitions between the Terrestrial and Epiphytic Habit Drove the Evolution of Seed-Aerodynamic Traits in Orchids
Wind also explains how orchids reach tree branches. Many tropical orchids grow on tree bark, not in soil, and only something as light as dust could reach those perches by air. The moth orchids sold in shops are this tree-dwelling type.
Other Plants With Dust Seeds
Orchids are not alone. Pale, leafless plants such as ghost pipe, in the heather family, also make dust seeds.22Seed Science Research. ‘Dust seeds’ with undifferentiated embryos and their germination in mycoheterotrophic Monotropoideae (Ericaceae) Most of their seeds are under 0.9 mm, with a food store of a few dozen cells or none.23Seed Science Research. ‘Dust seeds’ with undifferentiated embryos and their germination in mycoheterotrophic Monotropoideae (Ericaceae)
Ghost pipe is a white, waxy plant seen in shaded woods, with no green color because it makes no sugar from light. It gets its food through a fungus that is in turn linked to the roots of nearby trees, so it ultimately draws on the trees’ sugar. Its tiny seeds are a good example of a plant that relies on an outside food supply from the start.
Broomrapes and witchweeds are parasites that live on other plants’ roots, and they also make tiny seeds. Their seeds sprout only after sensing chemicals that leak from a host plant’s roots.24PubMed Central. The mechanism of host-induced germination in root parasitic plants The seed waits until a host root is close enough to tap.25PubMed. Convergent evolution of strigolactone perception enabled host detection in parasitic plants
How Long Orchid Seeds Survive in Soil
A seed bank is the stock of living seeds waiting in the soil. In field tests, seeds of some woodland orchids stayed alive in soil 4 to 7 years.26Biological Conservation. Seed longevity in terrestrial orchids – Potential for persistent in situ seed banks Seeds of rattlesnake plantain orchids, by contrast, sprouted or died within a year.27Biological Conservation. Seed longevity in terrestrial orchids – Potential for persistent in situ seed banks
Because some seeds stay alive in the soil for years, some orchids can return to a spot years after the parent plant vanished. If a wild orchid disappears from your woods, it may not be gone for good. Seeds in the soil can still sprout if the fungus is present, and leaving the ground undisturbed gives them the best chance.
Can You Grow Orchids From Seed
Yes, but in sterile jars, not by sowing them in a pot. In 1922 Lewis Knudson first sprouted orchid seeds on an artificial gel without any fungus.28PubMed Central. The challenges of growing orchids from seeds for conservation: An assessment of asymbiotic techniques Since then, about 270 orchid species have been grown this way.29PubMed Central. The challenges of growing orchids from seeds for conservation: An assessment of asymbiotic techniques With the best gel recipe, one threatened orchid sprouted about 95% of the time.30PubMed Central. An effective nutrient medium for asymbiotic seed germination and large-scale in vitro regeneration of Dendrobium hookerianum, a threatened orchid of northeast India Compared with under 1% in the wild, lab growing can save rare orchids.31PubMed Central. The challenges of growing orchids from seeds for conservation: An assessment of asymbiotic techniques
The gel contains sugar, minerals and a setting agent, and it supplies the carbon the fungus would normally provide. Because the sugar is available directly, the seed can use it without any fungal partner. Some recipes also add other nutrients that improve sprouting for particular species, which is why the best recipe varies from orchid to orchid.
Home seed growing means sterilizing the seeds first, since mold loves the sugary gel. Flasks are sealed and kept clean for months, and seedlings move to pots only once they have leaves and roots. Many hobbyists buy ready-grown flasks from nurseries instead.
Scattering seeds on potting mix fails because the mix lacks the partner fungus and the seed has no food to wait on. It dies before anything visible happens.
The Largest Seed at the Other Extreme
The world’s largest seed belongs to the coco de mer, a palm from the Seychelles.32PubMed Central. On the origin of giant seeds: the macroevolution of the double coconut (Lodoicea maldivica) and its relatives (Borasseae, Arecaceae) It measures 17 to 48 cm long and is said to weigh up to 25 kg.33PubMed Central. On the origin of giant seeds: the macroevolution of the double coconut (Lodoicea maldivica) and its relatives (Borasseae, Arecaceae) Against a 0.31-microgram orchid seed, that is roughly a hundred-billion-fold difference.
The two seeds do the same job in opposite ways. The coco de mer packs huge food stores so its seedling can grow in shade, and it makes few seeds that fall near the parent. Orchids pack nothing and send millions far away.