Spinescence in the pea genus Astragalus, part 2
(writing in progress)
The following makes sense once one becomes familiar with it. However, it is a permutation of spinescence that I did not dream of until I encountered it for the first time recently. This was one of those moments of ‘I wish I thought of that’ in the game of imagining ‘evolutionary options’ out of thin air.
Astragalus creticus is a hummock-like or cushion-like shrub associated with intensive grazing by domestic livestock for centuries, or more likely millennia, on the island of Crete.
It is no surprise that, being a pea (Fabaceae), the species has pinnate leaves that are drought-deciduous. And it is also no surprise that it is spinescent. Peas, after all, are often spinescent. Furthermore, drought-deciduous plants with pinnate leaves tend also to be spinescent in various families.
The surprises come in
- the organ from which the spines are derived (namely the rhachis), and
- the three-phase deployment of the spines, first as a foliar spine, then as a nodal spine, and finally as a caular spine.
It is unusual enough for a given plant species to possess more than one category of spine. But I have never previously encountered a plant that manages to pull off this combination using a single, versatile structure (by means of an ontogenetic shift).
And this, on reflection, seems such a good idea that immediately I find myself asking the following question:
Why would this pattern be restricted to Astragalus, and (as far as I know) absent from any plant in, for example, Australia?
My preliminary answer is that Astragalus – and particularly this species - represents an extreme of adaptation on Earth. Its habitat has been subject to intensive and consistent herbivory for millennia on what, by Australian standards, is a nutrient-rich substrate despite the stoniness.
Herbaceous plants have adapted to this sort of extreme herbivory by forming thistles. However, low shrubs have adapted equally extremely by using foliar spinescence in a different way.
Both thistles (which are unimaginably species-rich in the Mediterranean Basin although belonging mainly to the Asteraceae) and Astragalus derive spines from the leaves. However, what Astragalus does that is so novel to me is to use the rhachis in a way unfamiliar in any other plant.
The elegance of this adaptation lies in its simplicity: merely a functional shift by means of ontogeny, with a minimal shift in the form of the spine itself.
There may be parallels with Aciphylla in New Zealand, but I suspect that even that analogy may be limited.
Using the rhachis-tip as a foliar spine would be novel enough. But then keeping this rhachis after the leaf has died (or at least ceased to photosynthesise) on slow-growing stems with compressed internodes, and thus converting it into a nodal spine, is something akin to a ‘stroke of genius’ of Nature.
And then, just in case any really tough-minded ungulate (such as the donkey) gets it into its head to bite off a whole stem-system, the same spines can be retained as caular spines.
And all of this works because the plant, at the same time, adopts such a compact and ground-hugging form that herbivores have minimal access to the surfaces of the plant.
All of this certainly amounts to some sort of ‘Guinness record’ in the annals of anti-herbivore defences in plants. But an even more intriguing possibility is that Astragalus creticus has gone beyond defensiveness (fence-mindedness), to convert itself into something resembling a ‘pea-lawn’.
What I mean by this is a fabaceous plant that facilitates herbivory by ‘fencing off’ its precious shoots while allowing access to its leaves. Although so defensive, it may in some sense be ‘encouraging’ herbivory, as a means of competing with other plants in the indirect way grasses so often do. The hypothetical result is the management of herbivores so that they do not destroy its capacity to replace eaten leaves.
Please note that some photos show that A. creticus can virtually dominate the vegetation over small areas.
Please also bear in mind that Astragalus is well-known for its chemical defences, such as glycosides and selenium. The genus is valuable medicinally because of its toxins. And, if A. creticus retains such chemical defences, then it might be defended in an even more complex way than I portray here in the form of spines.
So my question is: is Astragalus creticus merely one of the most extremely defended plants anti-herbivory, in a habitat that is extremely herbivorous? Or is it the dicotyledonous equivalent of a shrub-gone-lawn, taking a principle seen in e.g. Vachellia and shifting it to such an extreme that it is hardly recognisable at first?
The following shows the foliage of Astragalus creticus in what I assume to be the rainy season in this Mediterranean climate.
In the following view of the foliage, the plant hardly looks spinescent.
The following shows the extreme growth-form taken by this species, which forms cushions or hummocks presumably in adaptation to an intense regime of herbivory by domestic livestock.
The following shows the plant in what I take to be the dry season (the Mediterranean summer) when the pinnae have fallen off. The plant certainly looks more twiggy than in the green season but even her its spinescence is not at first obvious.
http://mediplantepirus.med.uoi.gr/pharmacology_en/plant_details.php?id=167
The following close-up of the foliage shows the curved spines, which turn out to be derived from the rhachis.
The following close-up shows that the green leaf has a sharp tip to the rhachis, which is somewhat green while the pinnae are green. If one accepts this rhachis tip as a spine, the obvious classification of this form of spinescence is foliar spinescence.
The following again shows the foliar spinescence in Astragalus creticus, the spine being an essentially green rhachis-tip turned ‘pungent’.
https://www.greekflora.gr/el/flowers/0092/Astragalus-creticus
The following is what I take to be new growth, in autumn just after the arrival of the rains. Shoots are growing among the non-green spines, which makes these spines nodal spines. The big surprise is that these nodal spines are the very same spines seen above as foliar spines. The rhachis has been converted from a foliar spine into a nodal spine by the tactic of persisting after its pinnae have fallen, and retaining its spinescence once its green hue has faded. I do not know whether these spines remain alive, but this need not matter for its defensive function.
The following close-up again shows what are here functionally nodal spines, but what turn out to be persistent rhachises.
The following shows that the persistent spinescent rhachises persist way down the stem, which means that they persist for years in such a slow-growing shrub. This means that – wait for it – the same rhachis has now been converted to a caular spine!
The following two botanical paintings suggest that the caular spines can be green even low down on the stems, but I doubt this and I suspect that this botanical painting is inaccurate in this respect.
Here we have the caular spines depicted as non-green, which I suspect to be the truth.
The following two photos show again what the leaf is like when it first appears: it is a fairly normal pinnate leaf, typical of so many peas, except for a ‘pungent’ rhachis-tip that is probably more easily felt than seen.
http://www.cretanflora.com/astragalus_creticus.html
(writing in progress)
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