Tuesday, 17 February 2015

Wiping the glaucous sheen off a Pig's Ear


In early January I kayaked past this plant spreading up, or down, the banks of the Yarra River. Then a little later in the same day, walked past it in jutting from a rock wall, leaves sheening in the brilliant midday summer sun.

It's not a plant you can kayak or walk past easily, particularly when in flower. Cotyledon orbiculata, more commonly called Pig's Ear, is grey with a glaucous sheen, almost all over. The exceptions are the edges of the leaves, which are red, and the inside of the flowers, which in this case are orange to appeal to sunbirds.


Pig's Ear flowers mostly in winter but where there is more winter rainfall, such as the Western Cape (and here in Melbourne), in midsummer. Presumably the African sunbirds know when to be active.

According to Shireen Harris on the PlantzAfrica site, there are 10 species of Cotyledon, a genus name based on the Greek work kotyledon meaning cup-shaped hollow (and also used for what are called the seed-leaves of most flowering plants, the cotyledons).

The leaves of most species, including Cotyledon orbiculata, are cotyledon-like in shape.


The waxy white coating reflects the sun's heat, to prevent overheating and water loss. In low light, little wax is produced, in high light (such as on this rock wall), lots. When fully coated, up to 50% of the sunlight reaching the plant surface can be reflected and water loss reduced.


If you remove the coating, as I did here with my finger, it will take about two weeks to start returning again on that leaf. The waxy deposits are formed inside the leaf cells and exuded to the outside during growth and after injury. It seems that the route to the surface, and the means of production, do not shut down once the leaf is mature.

(I did a slightly more thorough test  on a smaller plant we are growing at home, wiping strips from two differently aged leaves to see what would happen. After two weeks a light cover of wax had returned on both, and now, just over six weeks later, the coating is thick enough to make the strip look grey rather than green, It's still noticeable though and I wonder if it ever completely recovers its full coating.)

I doubt that in the Australian summer my interference will cause the plant, or even this leaf, to die (the cuticle, a more important waxy-outer-layer on nearly all leaves, did survive my vandalism), but is a shame to do it more often than you need to in the interests of curiosity. If for no other reason, it detracts from the beauty of the plant as a whole.

In their southern African homeland, in rocky outcrops, the plant may well be more finely tuned. One poorly performing leaf could be the difference between living and flowering, or death. Perhaps. 


For me now, there are those complex feelings after you have done some thing you really shouldn't have. I know this leaf will recover its wax but will it be in time to survive the summer? I'm not sure if it matters, given the small surface I scraped, but I do add it (along with memories of insects I've squashed, birthdays I've forgotten and chances of selfless assistance to others) to my life-time load of guilt.

But then, in Australia Pig's Ear has escaped from gardens and is spreading through bushland areas, such as the Yarra River verge near my home. If I were to think instead about the potential of this plant to jump the garden fence and spread further into native vegetation, displacing indigenous plants and animals, I could convince myself this was a (very) small contribution to conserving Australia's biodiversity.

But that would be stretching things a little (a lot). And unfair to my neighbour's garden.

Images: all from the plants growing out of the rock wall in a nearby street. 

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