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Ken Thompson: what a winter twig can tell you (if you know where to look)

Buckthorn
Original description:	Hippophae rhamnoides 
Buckthorn (Hippophae rhamnoides)  Credit: Simone Augustin/ GAP Photos

It’s not every day you open a book that genuinely expands your horizons, but that’s what happened when I opened my new copy of John Poland’s Field Key to Winter Twigs (from NHBS, £19.99). Let’s be honest, winter can be a dull season for the gardener; there’s a limit to the amount of time you can spend reading seed catalogues. But here is a whole new winter activity that hardly involves going outside at all, and can mostly be carried out in an armchair in front of the fire, cup of tea and chocolate biscuit at your elbow.

All you need is some young twigs, a decent hand lens and a ruler, and you’re ready to sort out all those annoying trees and shrubs in the local park or road verges. OK, I know there’s nothing to stop you doing that in the summer, using the more traditional flowers, leaves, etc, but there always seems to be something better to do in the summer, and it turns out that bare twigs are surprisingly informative.

There’s quite a bit of botanical detail to cope with, so full marks for Poland’s attempts to come up with some memorable images to help us on our way. Like the “hula skirt” papery sheath at the base of Japanese maple buds, or the sharply curved up and down lower branches of horse chestnut. “Like an elephant’s trunk – imagination required!” says Poland, and I can see what he means on both counts. As for the “monkey-face” leaf scars of walnuts, the description is spot on – once seen, never forgotten.

Even so, I admit you have to learn quite a bit of new jargon, and you also have to accept that some familiar words have precise botanical meanings that you ignore at your peril. How many of us, for example, have ever given much thought to the difference between prickles, spines and thorns? Not many, and maybe you’d still be happy not to, but I’m going to enlighten you anyway.

Prickles are simply an extension of the plant’s skin, or epidermis, and can occur anywhere on a twig or branch. Roses and brambles are the classic examples. Spines are modified leaves or stipules (bracts, sometimes very leaf-like, found on the petiole/leaf stalk, or on the twig at the base of the petiole). In either case, spines occur directly below a leaf scar or a bud, since buds are always in the angle between leaf and stem. Robinia, berberis and gooseberry (and cacti of course) all have spines.

Thorns, on the other hand, are modified branches and therefore always occur above a leaf scar or at the end of a short branch. Thorns can terminate a main or a side twig and (because they’re branches) they can bear buds and leaf scars of their own.

Contrary to the normal dictates of sod’s law, buckthorn, hawthorn and blackthorn really do have thorns, and neither prickles nor spines (so whoever named blackthorn Prunus spinosa was having a laugh).

Since roses don’t have thorns, does that mean “a rose between two thorns” is wrong? I don’t think so; the allusion is surely to the plant rather than the flower, so it’s a rose (bush) between two thorn (bushes).

Ken Thompson is a plant biologist with a keen interest in the science of gardening. He writes and lectures extensively; his most recent book is Darwin’s Most Wonderful Plants. Visit books.telegraph.co.uk