confronting vegetation: Petrophile sessilis

Sometimes I have to bring an Australian plant home, and live with it for a while, before I start seeing how truly alien it is.

 

Petrophile sessilis grows on open sandstone heaths around Sydney, generally exposed to high winds.  Sandstone soils are even poorer in nutrients than average Australian soils, and also don't retain water for plants very well.  Petrophile is often the tallest plant in these heaths, at up to 3m.

(A testament to its toughness is that this particular cutting has been sitting in water for almost a month, without losing much of its color. One survives in tough and stormy spots by not reacting quickly to change.)

Like many Acacia species who also like tough sites, Petrophile is leafless. It photosynthesizes through its bare green stems.  This is an adaptation to fierce drying winds.  (Leaves reduced to needles are also common in Australia, and serve the same purpose.)  But it's most curious character takes a moment to identify.  There's a nice moment when the plant seems confounding but you can't quite say why.

Petrophile is a divaricating shrub.  In most shrubs, it's typical for a stem to have leaves or lesser stems coming off the side, but there's usually a clear distinction between the main stem and the secondary branching ones.  Here, by contrast, as a stem grows away from the reddish "trunk" it periodically splits into two stems, which head off at about 60 degrees to the right and left.  These stems continue to split periodically, creating a three-dimensional space shaped by this endless dividing.  It's also important that the stems don't get thinner as they split -- as they would in many plants -- so that the whole mass seems made up of just one kind of stem. 

Because each split is even, the plant is full of 120 degree angles, and the hexagons that this angle implies.  But unlike, say, the Gleichenia fern that I photographed in Tasmania, the segment-lengths vary, so the hexagons are never perfect, more a subtle suggestion of principle in the complexity of the real.

In a sense, Petrophile is a diagram of the most confounding idea underlying the theory of evolution, that unfathomable complexity can arise from simplest rules. 

Although Petrophile is Australian, divaricating shrubs seem to be especially common in New Zealand.  Why would divarication evolve among different plant families in the same isolated place?  Did these fractal patterns somehow confound the moa, so that these shrubs were less likely to be eaten?  Or is there some other genius loci at work? 

Twenty years ago I'd have wanted to answer these questions.  Now I just want to touch them, look at them from different sides, put them in water and live with them for a bit.

Comments

[Teresa Gilmant]

Something that I knew as a child was designed after the shape of this plant. It's nagging at the edges of my memory, but I can't quite get at it. What ? When ? It is made of glass, I think. And pertains to winter. This is all I've been able to dredge up. What a soup memory is. What a treasure/curse.

You don't care about the answers, just the touch. Absolute poetry.

Jarrett, I'm glad you posted today. I was about to send you a message that said: You haven't posted in longer than I haven't; I hereby challenge you to a duel. (or maybe a blog-off). ;-)

Now I have NO excuse.

Teresa

[dale]

wonderful post. Thank you.

[Miss Bliss]

I would call that progress my friend.

[Philip]

I've missed your eye on those nature rambles. The weekend before last I did that walk in Dharawal SCA that I had originally planned to do with you, and was reminded strongly of that fact by the abundant Petrophile sessilis at the starting point. It was a lovely day, with lots of interesting plants in flower, although not as lovely as Ku-ring-gai Chase due to the still obvious mark of a fire from a few years back. I found myself wondering what was different about this sclerophyll woodland that caused it to be dominated by Petrophile sessilis where Ku-ring-gai Chase is all Petrophile pulchella. Was there a geographical separation going on here? But a few kilometres in, just as I was getting used to the strangeness of Petrophile sessilis again, an individual plant caught my eye that I realised was P. pulchella. Within a hundred metres or so, the composition changed until all the Petrophiles were P. pulchella, with no immediately obvious difference in the substrate. Curious.

[Jarrett]

Philip, with whom I go botanizing almost every weekend when I'm in Sydney, is too polite to correct me on the record, but I admire his precise insistence that I was wrong to speak of Petrophile's divaricating stems. Technically, everything but the central stalk is a leaf.

Thanks for the comment, Phil. Look forward to seeing you in New Caledonia.

J

[Philip]

I find the idea that I am polite curious. I was inspired by this post to have a look at the Flora of Australia volume on Proteaceae, including Petrophile, a gift of a zoological friend who on receiving it for work, forwarded it onto me, stating "I think you'll get more out of this than I would." The 5 eastern Australian species have rather similar leaves (I'm using leaves in a botanical sense as Jarret has noted, to mean the entire complex divaricate structure emerging from the stem), albeit with Petrophile sessilis being the most bizarre in the extreme angles between the successive veins of the lamina. However, amongst the 48 species confined to southwestern WA, the leaves follow the full gamut of shapes from simple flat leaves, to progressively more divided leaves, followed by leaves where the leaf is essentially just veins, as in Petrophile sessilis, and finally species where the leaf is just a simple needle. Seeing them all gathered together is something like seeing evolution in action, and goes to back up what I have always said, which is that if you want to get a true appreciation of the diversity and strangeness of the Australian sclerophyll flora, it's pretty much compulsory to visit the southwest botanical province of WA at some point.

Of course, Murray Henwood, one of our lecturers at Sydney University, originally a New Zealander, used to always say that every Australasian botanist must visit New Caledonia at some point, so I look forward to many startling sights there.