Friday, April 3, 2015

Dichotomous key (VI) - A nod to nodes before moving on to the space between

So when not looking at the buds themselves, there's a lot that a branch can tell us both in terms of what we're looking at and the ecology of the plant. A node is a point at which a bud (and subsequent leaves, fruits, branches) emerge from a twig. Depending on the branching pattern of the twig (phyllotaxy describes arrangements of leaves on a stalk) there can either be one, two, or a whorl of buds, etc. emerging from a given node. If thar be one, tis alternate, but if a pair as in two, then opposite, and any mar it be a-whorled with branches (as in many conifers and catalpa). 

The space along the branch or twig between each node is called the internode. Rather than an empty desert stretching the gap between our beautifully relevant little buds, who are oh so helpful in identification, the internode is rich repository of information about the twig and the ecology of the tree. This post started as just one, but quickly became three.

I wrote earlier about sub-opposite branching patterns, the forth option to bud arrangement at a node. One of the things I had read in Peter Thomas's Trees: their natural history, which I can't recommend enough, is that in opposite branched trees the faster that tree is growing the more likely its buds are to be slightly offset from one another. This is often readily apparent in common buckthorn branches, where two buds are slightly offset from one another followed by a much wider gap before the next pair of offset buds.

White ash, typically opposite, showing distinctly alternate branching at the tips of its rapid growth

In an open field at Rock Point, one which receives regular hair cuts every few years, there are a number of white ash saplings growing. The trees probably grow for several years, get cut back, and then sprout up again from the same root stock. With an abundance of stored energy, the new growth is extremely vigorous. I found the above seemingly alternate branched twig on one such sapling. Pretty impressive that the growth was fast enough to result in opposite buds offset enough from one another that this distance was equal to the distance between the nodes!

When thinking about ultimate causes of branching patterns, it is a tricky nut to crack. Researchers studying phyllotaxy - or the spatial arrangement of leaves - of rice, wrote in their paper: "the mechanism responsible for this extremely regular pattern is one of the most fascinating enigmas in plant biology." So while we may not fully know what controls phyllotaxy, observing patterns of conditions that result in changes to a plants norm is beginning to illuminate what factors impact phyllotaxy (Here's another really fascinating article about auxin's role in controlling growth patterns in plants, not just with branching).

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