Tuesday, 22 September 2015

Magnolia reveals leafy road map

For much of the twentieth century there were a dozen genera in the family Magnoliaceae, with the namesake Magnolia the best known. You may also have heard of Michelia (including the Port Wine Magnolia) and Liriodendron  (the two Tulip Trees).

As the Magnolia Society International explain, the number of genera was reduced back to seven in the 1980s although, apart from the Tulip Trees which everyone accepts as distinct, this system was not accepted universally. Not to worry though, the advent of DNA sequencing led to an even better understanding of the family and evidence that all the smaller genera apart from the Tulip Trees were embedded within the family tree of Magnolia. Now there are two genera only, one with 13 sections (there could have been 13 genera but the genetic differences were small, the identification of some groups difficult and it would have resulted in even more disruption to names).

The flowers of Magnolia no matter how you circumscribe it, are what you first notice. Big, bold and full of stamens (male bits) wrapped around a cluster of carpels. This used to be considered a 'primitive' flower structure but that mantle is now bestowed on the rather simpler flowers of plants like Amborella (although the whole concept of a flower today being primitive is not very meaningful).

It wasn't the pillowy white flowers of this species of Magnolia that grabbed my attention this time but something more subtle, something I only noticed when I was photographing the leaf to help me identify the species, Magnolia doltsopa (once Michelia doltsopa) I believe.

I noticed first the leathery upper surface, but then the intricate patterning of the veins. This is what we call reticulate, or networked, venation. Like a net.

In this case, and it's not always the case, he final threads are 'free' - not connected to anything. So instead of it being a web, there are dangly bits inside each of the smallest defined areas (called areoles or areola). It's like an irrigation system, getting the drips or sprinklers evenly spread so the water can reach as much of the area as possible.

In a way this is what the veins do too. They are more technically called vascular bundles, and they do act as a kind of vascular system similar to the one that carries blood through our own bodies. Instead of blood the bundles carry water (and nutrients) from the roots to the leaves where it is needed as part that vital chemical reaction photosynthesis, and then the energy-rich products of that process, sugars, to where they are most needed in the plant. There are two sets of 'pipes', the xylem carrying the water and the phloem carrying the sugars.

What you see in a leaf are the outlines of this transport system. The main veins are the first thing you notice. They are usually parallel in what we call often call Monocots or Monocotyledons - grasses, lilies and the like - and more often spreading out from a main central vein in the Dicots with their mostly broad leaves. There are lots of variations and in fact it can be a useful identification character.

In this species the patterning is the same on the top and bottom, with just a different sheen. The top is brown and leathery, the underneath whitish with a tint of blue.

To identify this particular Magnolia (as a Michelia as it happens) in the Horticultural Flora of South-Eastern Australia (Volume 2) by Roger Spencer, I had to count the main veins and confirm there were less than 12. I didn't have to look to any of the finer detail in this case but sometimes that's important too.

Putting aside its value to the plant and to taxonomists (those who name and identify plants), the venation is just a fascinating construct. Clearly it's a good way to maximise the reach of your plumbing and transport system, as any property developer knows...

Update 27 September 2015: See latest from Royal Botanic Gardens Kew on new chemicals discovered in Magnolia.

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