Sunday, February 21, 2016

Alternate or Opposite

When I’m talking to people about plant identification, leaf arrangement is the first place that I start. And while I’m usually talking about woody plants (trees, shrubs and vines), leaf arrangement on perennials is used for identification as well. To simplify the concept, this post will focus on woody plants.

Calycanthus floridus, opposite leaf arrangement
Leaf arrangement on a plant is either opposite or alternate: leaves may be opposite one another on the stem or they may be alternate along the stem.

Occasionally you might find plants with leaves arranged in a whorl, which is 3 or more leaves together. Lily (Lilium) is a good example of a plant with leaves in a whorl.

Viburnum with opposite twigs

In the winter time, you might think that you can’t discern leaf arrangement on deciduous plants because the leaves have dropped. The clues are still there, even when the leaves are gone. In younger trees or shrubs, you can study the branches – they follow the same pattern as the leaves. Older trees might not have all their original branches so it’s best to examine the twigs.

The twigs are the ends of the branches; they represent the most recent year’s growth. In a slowly growing plant, the twig might be very short, but in a rapidly growing plant there could be several inches of twig. The fresh growth on the twig is a good place to check for leaf buds.

Hawthorn (Crataegus) leaf buds, alternate

Leaf buds are next year’s leaves. They may be large and prominent like American beech (Fagus grandifolia), or they may be so small as to appear hidden like sweetshrub (Calycanthus floridus). In both cases, you can still discern the arrangement.

Most leaf buds are large enough to see in the winter. You will usually see the leaf bud and below it is the leaf scar (where last year’s leaf was attached). Only occasionally do I feel the need to use a 10x hand lens to figure out arrangement. Another tip: twigs with opposite leaf arrangement are fairly straight while those with alternate leaf arrangement might have a bit of a zigzag appearance.

Beech leaves, alternate
Leaf buds are not visible in the spring and summer because they haven’t formed yet, but it doesn’t matter because we have the leaves to help us to see the arrangement. Occasionally some leaves are missing, so look carefully before you decide the arrangement is alternate; it might be opposite with missing leaves.

Now that you’ve identified the leaf arrangement, you can proceed further into identifying the plant. Most plants have alternate leaf arrangement, so if yours has opposite then you have a smaller set of choices.

I’ve given up on using mnemonics to remember the common opposite plants; every version I’ve heard leaves several of them out. Here are the common ones in Georgia: maple (Acer), buckeye (Aesculus), Viburnum, ash (Fraxinus), dogwood (Cornus, except one species is alternate), elderberry (Sambucus), fringetree (Chionanthus), beautyberry (Callicarpa), Euonymus, honeysuckle (Lonicera), sweetshrub (Calycanthus), and buttonbush (Cephalanthus).

Most identification books offer a “key” for you to follow. The key walks you through a series of questions until the plant is identified. Winter keys will focus on other twig characteristics such as leaf scars, bundle scars, terminal bud and bud characteristics and presence of lenticels (you might want that hand lens here). Summer keys will focus on the leaf characteristics as well as flowers and fruit (seeds, nuts, berries).

I always recommend that the best way to learn is to practice on plants that you already know until you are familiar with the keying process.

Good resource: Native Trees of the Southeast by Kirkman, Brown and Leopold (older version of this book is Trees of Georgia and Adjacent States).

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