Orchid seeds

My ankle brushed against the dried flower stalk of a cranefly orchid and puff! a cloud of sandy dust billowed across the surrounding leaf litter. I got down on the ground for a closer look: the orchid’s fruit capsules were mature and starting to split apart.

Each capsule is roughly the size of a pinto bean. Inside are thousands of seeds. To the naked eye the massed seeds look like piles of very fine sawdust; with a squint we can make out the individual seeds. A camera lens and digital zoom lets us see a little closer.

Cranefly orchid leaf with its distinctive purple underside. The leaf appears in fall then dies in the spring.

Cranefly orchid leaf with its distinctive purple underside. The leaf appears in autumn then dies back in the spring.

Cranefly orchid capsule, split open and shedding seeds.

Cranefly orchid capsule, split open and shedding seeds.

Thousands of seeds in one capsule.

Thousands of seeds in one capsule.

A tiny puff of air is all they need to take flight.

A tiny puff of air is all they need to take flight.

These seeds owe their existence to pollination by noctuid moths. The moths suffer the indignity of carrying orchid pollen on their eyes. The cranefly flower has a slight twist and the direction of this twist determines whether the left or the right eye of the moth receives the pollen.

The wind-blown seeds’ future depends on where they land. Successful growth requires (or is greatly helped by) the presence of decomposing wood, so this orchid is one of thousands of species in these forests that depend on old logs and fallen branches.

Regular readers of Ramble will be interested to know that this orchid’s only close relatives live in east Asia. It joins many other plant species in reminding us of the ancient connections between the forest of the southeastern US and those of eastern Asia.

The abundance of orchid seeds has impressed botanists for centuries. Here is Charles Darwin calculating that one plant could in a couple of generations of unchecked seed production “clothe with one uniform green carpet the entire surface of the land throughout the globe.”

“[seeds] are produced by orchids in vast profusion. Not that such profusion is anything to boast of; for the production of an almost infinite number of seeds or eggs, is undoubtedly a sign of lowness of organisation, … a poverty of contrivance, or a want of some fitting protection against other dangers. I was curious to estimate the number of seeds produced by some few Orchids; so I took a ripe capsule of Cephalanthera grandiflora, and arranged the seeds on a long ruled line as equably as I could in a narrow hillock; and then counted the seeds in an accurately measured length of one-tenth of an inch. In this way the contents of the capsule were estimated at 6020 seeds, and very few of these were bad; the four capsules borne by the same plant would have therefore contained 24,080 seeds. Estimating in the same manner the smaller seeds of Orchis maculata, I found the number nearly the same, viz., 6200; and, as I have often seen above thirty capsules on the same plant, the total amount would be 186,300. As this Orchid is perennial, and cannot in most places be increasing in number, one seed alone of this large number yields a mature plant once in every few years.

To give an idea what the above figures really mean, I will briefly show the possible rate of increase of O. maculata: an acre of land would hold 174,240 plants, each having a space of six inches square, and this would be just sufficient for their growth; so that, making the fair allowance of 400 bad seeds in each capsule, an acre would be thickly clothed by the progeny of a single plant. At the same rate of increase, the grandchildren would cover a space slightly exceeding the island of Anglesea; and the great grand-children of a single plant would nearly (in the ratio of 47 to 50) clothe with one uniform green carpet the entire surface of the land throughout the globe. But the number of seeds produced by one of our common British orchids is as nothing compared to that of some of the exotic kinds …  What checks the unlimited multiplication of the Orchideæ throughout the world is not known.”

(p. 277-279 in Darwin, C. R. 1877. The various contrivances by which orchids are fertilised by insects. London: John Murray. 2d edition, quote from the Darwin-Online archive.)

13 thoughts on “Orchid seeds

  1. Jim Markowich

    Splendid observation and insight ~ thanks!

    So 135 years later, do we have any better understanding of what checks their unlimited multiplication? I know that foxglove also produces a zillion (well, more or less) seeds per plant, yet even when they grow in some profusion in a corner of the yard, that means only a few dozen plants.

    Reply
    1. David George Haskell Post author

      I suspect that we have a little, but not a lot, more insight. Landing in the right place for successful germination must be the first hurdle: the paper on decomposing wood suggests that the species needs particular conditions (and not doubt mycorrhizal fungi to help out).

      Reply
  2. Todd Crabtree

    I have seen Tipularia tubers growing half exposed on crumbling logs. This is the closest thing we have in Tennessee to an epiphytic orchid. Sometimes a small colony of plants can be seen pushing up through the leaf litter in a well defined line, an obvious sign that they are utilizing a fallen branch that has been subsumed on its way to becoming soil. I saw this pattern again beside the trail to Walls of Jericho State Natural Area this past Saturday.
    The mysterious relationships between orchid seeds and fungi have not been thoroughly studied but some details have been revealed. It is known that some species of orchid also benefit from this relationship as mature plants and the presence of the fungi might allow the orchids to survive in marginal habitat.

    “Recently, the fungus species Epulorhiza inquilina was identified (Currah et al. 1997) and has been shown to induce seed germination of P. integrilabia in vitro (Zettler and Mclnnis, 1992).”
    Genetic Diversity and Microsite Characterization of the Rare “Monkeyface Orchid”, Platanthera integrilabia (Orchidaceae), in the Southeastern United States. Unpublished thesis. Inna V. Birchenko. November 2001.

    Currah, RS, LW Zettler and TM McInnis. (1997). Epulorhiza inquilina sp. nov. from Platanthera (Orchidaceae) and a key to Epulorhiza species. Mycotaxon, 61, 335-342.
    http://hdl.handle.net/10402/era.30866
    Zettler, LW and TM McInnis, Jr. 1992. Propagation of Platanthera integrilabia (Correll) Luer, an endangered terrestrial orchid, through symbiotic seed germination. Lindleyana 7: 154-161.

    Reply
    1. David George Haskell Post author

      Hi Todd, Thank you for these references and great observations. I’ll keep my eye open for them on logs (fresh ones and older ones). They seem to get onto old road beds too (e.g., very old wagon and logging roads around Sewanee). Not sure what the mechanism would be there — maybe exposed soil with fallen wood? Many thanks, David

      Reply
      1. batesvillian

        David — Cranefly orchids are common in the young(ish) woods surrounding our house in piedmont Virginia. This is just speculation on my part, but perhaps they are one of those species that actually benefits from, or at least isn’t harmed by, disturbed forest habitat.

        Their presence along old logging roads suggests that the species managed to hold on during successive clear cuts and selective thinning by colonizing the decomposing detritus (slash piles, decaying stumps, lots of sawdust and wood chips) that goes along with otherwise destructive logging practices.

        Combine that with promiscuous seed production and the continued existence of its insect pollinator, and you get a species that could essentially leapfrog out of forest remnants into successively aged stands of trees.

        Might be interesting to examine orchid density as it compares to forest age…
        William

        PS — Despite the cranefly orchid’s ubiquity, I still love to see them in the woods. Their tiny flowers are also beautiful, as I think you’ve noted in past Ramble posts.

        PPS — Woodland gardeners with a few cranefly orchids can greatly increase their numbers by letting fallen trees, limbs, and branches decompose naturally. If you have to grind up a recently fallen tree, spread the wood chips around or use them to make a path. You’ll soon find it lined by the orchids.

        Reply
  3. Brad Stroup

    I am impressed by the profligacy of these orchids, as I am of many plants’ efforts to propagate. And also the explanations we have for why so few seeds make it — poor soil, drainage, light, etc. But being a human of limited mind and scope, I have to wonder at these plants’ vast wasted efforts. After so many failures, why don’t they learn from their failures and focus their energies into more effective efforts for their offspring? After so many ions, surely some of them have learned something. Is it because they keep hoping for better times to take over the earth?

    Reply
    1. David George Haskell Post author

      Hi Brad, Good questions. Perhaps profligacy is the best strategy for these plants: produce LOTS of little seeds rather than just a few large, well-nourished ones? A bit like the difference between a ragweed and a coconut, or between rats and elephants? Different environments call for different strategies.

      Reply
      1. Brad Stroup

        David: Many thanks. I’m really asking a biological question as much as botanical one: Won’t a profligate plant some day adjust its wasted energy in its environment as a animals eventually do?

        Reply
        1. David George Haskell Post author

          Not sure I understand the question.

          There are generally two extremes (and most species fall somewhere in between: make a small number of offspring and invest very heavily in each one OR make lots and lots of offspring and invest very little in each one. In theory the second strategy works best when the payoff is highest for having more kids rather than better provisioned kids.

          Reply

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