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Apr 08

Organism of the week #11 – Not even primitive

It’s not much to look at, but Psilotum nudum‘s naked fronds hold a cautionary tale for biologists:

Psilotum nudum (Oxford Botanic Gardens) [CC-BY-SA-3.0 Steve Cook]

Psilotum nudum, finally bagged at the Oxford Botanic Gardens

Psilotum looks a great deal like some of the earliest fossils of land plants. One of the earliest such fossils is Rhynia, the first specimens of which which were unearthed in Aberdeenshire around 1910, a mere 410 million years after they were originally buried.

Rhynia gwynne-vaughanii reconstruction [CC-BY-SA-3.0 Griensteidl, based on Kenrick, P & Crane, P. R. (1997) The origin and early diversification of land plants. A cladistic study. Smithsonian Institution Press, Washington and London, fig. 4.8, p. 101]

Rhynia gwynne-vaughanii reconstruction [CC-BY-SA-3.0 Griensteidl]

Psilotum and Rhynia are anatomically quite similar, despite the latter being 410 million years dead, and the former being very much still alive.

They both have a horizontal stem called a rhizome, from which upright stems grow. These upright stems branch into two equal sub-branches again and again and again in both species. Both plants carry reproductive structures called sporangia (below) on these branches. They both have relatively simple plumbing inside their stems, and they lack true roots and leaves, although both plants appear to be associated with symbiotic fungi.

Psilotum nudum sporangia [CC-BY-SA-3.0 Eric Guinther (Marshman@Wikipedia]

Psilotum nudum sporangia [CC-BY-SA-3.0 Eric Guinther Marshman@Wikipedia]

As a result of these similarities, Psilotum has often been fingered as a primitive relic of a bygone age, and – until someone corrects it – its Wikipedia entry will continue to claim:

Psilotum nudum, […] is considered a “primitive” plant – a descendent of possibly the first group of vascular plants which were widespread during the Devonian and Silurian periods.

Interpreting fossils is not at all easy, and this is not a gripe at palaeontologists. However, this tendency to see some organisms as primitive relics, and others (generally the ones we eat, or the ones we are) as advanced, is as tempting as it is wrong-headed, and it needs to be resisted. All land-plants are descendents of the first algae that started made a living on land during the Devonian, this is not a singular characteristic of Psilotum, or whatever poor organism has been tarred with the primitive brush this week.

In Psilotum‘s case, not only is dismissing the plant as a primitive relic wrong-headed, it’s completely misleading. Psilotum is not a primitive relic, because the whole concept is nonsense; but it’s not even a primitive relic on this concept’s own nonsensical terms.

Recent molecular data place Psilotum not as a “primitive” or “basal” out-group to the rest of the land-plants, but simply as an odd-looking fern.

Fern phylogeny [CC-BY-SA-3.0 Steve Cook with images from Alex Lomas, Peter Woodard and Abalg]

Fern phylogeny [CC-BY-SA-3.0 Steve Cook with images from Alex Lomas, Peter Woodard and Abalg]. Asplenium and Marattia repreent the two major lineages of ferns-that-look-like-ferns-ought-to, Ophioglossum is an adder’s tongue fern, Equisetum is a horsetail. This phylogeny differs slightly from the Vegetable Empire 2013 I posted last week in the placement of the horsetails. Such is the nature of this fast-moving field.

Psilotum‘s resemblance to fossils like Rhynia and Psilophyton (below) is illusory. It is a modern organism nested within a group of plants whose most recent common ancestor had leaves and roots and all the other trappings of fern-hood. Psilotum is not primitive, because the word is meaningless when applied to species (and to more inclusive taxa). But even if we did accept that some plants are inherently primitive, Psilotum doesn’t fit the bill: its seemingly primitive leafless and rootless state turns out not to be the result of retaining these features from early land plants like Rhynia, but of secondarily losing leaves and roots possessed by the common ancestor of all ferns.

Psilophyton dawsonii [CC-BY-SA-3.0 Ghedoghedo; modified by Peter Coxhead]

Psilophyton dawsonii fossil [CC-BY-SA-3.0 Ghedoghedo; modified by Peter Coxhead]

The cautionary tale of the Psilotum nudum is just one example of many face-omelettes that Nature has served up, and will continue to serve up, for as long as anyone thinks of any organism as the primitive relic of a bygone age.

Can we please let the nonsense word ‘primitive’ itself become a relic of a bygone age?

Pretty please?

With sporangia on top.

2 comments

  1. Fran Turrubiates

    Horsetail contains silicon, which plays a role in strengthening bone. For that reason, it is sometimes suggested as a treatment for osteoporosis. It is also used as a diuretic, and as an ingredient in some cosmetics. However, very few studies have looked at horsetail’s effect in humans.”,*,

    Look out for our very own web blog too
    http://www.healthwellnessbook.comcr

  2. polypompholyx

    Do please post a link to a paper (or preferably, to a meta-analysis of trials) associating dietary supplementation with silicon to reduced levels of osteoporosis. I can find none via Cochrane or NCBI, but I presume you would not make such a claim without clear evidence.

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