#4 Banksia marginata – the lonely Proteaceae

IMG_9732_500gBanksia marginata, common name Honeysuckle referring to the large amounts of bird-attracting nectar produced by the golden flower cones. Also known as Coastal Banksia, and Silver Banksia, in this case referring to the off-white hairy undersurface of the leaves. The Banksia genus was named in honour of Sir Joseph Banks. Species name marginata comes from Latin “marginatus”, meaning “with a border”, referring in botany to a distinct margin. This is one of only two Proteaceae species recorded on Maatsuyker Island,  (the other is Cennarhenes nitida, Native Plum), leading me to ponder why the Proteaceae family is so poorly represented. At home in the Blue Mountains where Proteaceae species are common, the sandstone soil is nutrient-poor. On Maatsuyker Island, large areas of soil cover are affected by the guano of the colonies of seabirds.

“As already noted, the diversity and abundance of Proteaceae increase from fertile to infertile habitats. Indeed it is only in infertile scrubs and heaths in the region that Proteaceae may completely dominate the canopy.”
Ecology of Proteaceae with special reference to the Sydney region
P.J. Myerscough, R.J. Whelan and R.A. Bradstock

Cunninghamia 6(4): 951–1015

This winter (2018), I have been studying the 11 Banksia species – including B. marginata – of the Blue Mountains where I currently live. Many Banksias flower through winter.  Their golden yellow/orange cylindrical spikes light up the bush and their nectar attracts many birds, including honey-eaters, and insects. The floral parts of the members of the Proteaceae family are unique and fascinating, so let’s delve deeper, specifically with respect to the Banksia genus.

The most common arrangement of reproductive parts in the plant world include female parts (aka pistil) – a single stigma, sitting atop a style, which receives pollen, and male parts (aka stamen) – several or many anthers, each sitting atop a filament, which produce pollen. However,  things happen a little differently in the Proteaceae family, including Banksias.

BanksiaSpinulosa_smThe easily recognised Banksia “flower” is a cylindrical cone-shaped “inflorescence”, or cluster of flowers; not just a single flower but hundreds of flowers, uniquely arranged. The photo on the left shows a B. spinulosa growing in my garden.

Careful inspection of the two cones reveals that they are at different stages of development. The foreground cone, shown below on the left, is a cone still in bud, while the background cone, shown below on the right, has more developed flowers. “Anthesis” is the process of the flower developing from bud stage to being fully open and functional, and in Banksias, this is very obvious because the style (female part) can be seen to extend way beyond the rest of the floral parts.

banksiaspinulosa_budcone.jpeg  BanksiaSpinulosa_LateCone

(The styles here are the deep purple “hooks”).

BanksiaMarginataCloseup_sm.jpegIn this close-up photo (left) of a Banksia marginata from my garden, you can see the flowers occur in pairs. And this is a cone still in bud, pre-anthesis, the styles have not yet fully emerged from the “perianth cup”. The perianth cup encloses 4 anthers, the stigma and the “pollen presenter” (more on that later!).

Usually the flowers develop from the bottom of the cone to the top, and if you look closely at the lower flowers, you can see the pale-green emerging styles.

Here are some photos of the emerging (purple) style from the Banksia spinulosa, taken with my USB microscope camera.

(1)BanksiaSpin_EmergingStyle1_sm   (2)BanksiaSpin_EmergingStyle5_sm   (3)banksiaspin_emergingstyle4_sm.jpg

Several families, including Proteaceae, have a highly-specialised floral structure called a pollen presenter, from which pollen is distributed (as opposed to the more common arrangement of pollen being produced and distributed in the anthers).  Pollen is deposited on the pollen presenter, an area at the tip of the style, pre-anthesis, i.e. while it is still enclosed in the perianth cup. Banksias avoid self-fertilisation, by keeping the pollen out of contact with the stigma.

 

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This entry was posted in Native Plants of Maatsuyker Island and tagged , , . Bookmark the permalink.

One Response to #4 Banksia marginata – the lonely Proteaceae

  1. Pingback: My Maatsuyker Island Plant List | MaatsuykerDreaming

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