A Neotropical Savanna

Suddenly, yellow is everywhere. I’m seeing the Mexican sunflower, Tithonia, in many more places this year than last year; and the spectacular buttercup tree, Cochlospermum, is beginning to show in some places. But in our savanna, right now, the showy yellow tree is a living fence – and member of the legume family, Fabaceae. It’s Diphysa americana, locally known as macano.

And when you get up close to the flowers on a sunny morning, their exuberance can take your breath away.

The petals drop off quite easily, leaving a yellow snow on the ground.

Macano trees are part of the living fences in our area, although other species also are seen. Post-and-wire fences around here are really “stick” or pole-and-barbed-wire fences. Many of the poles are of macano wood. Soon (often within a year) after a pole is sunk into the ground, it starts sprouting.

Here’s rectangle A, showing the shoots, and the barbed wire attached to the pole:

Rectangle B shows the arrangement of the leaves, which are typical of the bean family. We’ll return to that arrangement later.

Macano trees are slow-growing but may reach a height of 20 meters (> 60 ft). The wood is strong and valued not only for fence posts but for tool handles, carvings, and multiple other uses. In fact, it is so treasured, that recently when we had ordered macano posts for a new fence (to keep our dog in and neighboring horses out), the order could not be completely filled because the wood had become unavailable. When it became available again, it was at a price 150% of the original!

So, I was not surprised to notice one day that a post that had grown into a mature tree had itself been harvested for wood:

Notice the splintered top of the post, showing how recent the harvest is, and notice that the bottom three rows of barbed wire have been overgrown by the tree. The top two rows were obviously added later. (These features are easier to see if you click on the image to enlarge it.)

Let’s look at why the macano is in the legume, or Fabaceae, family.

Third Largest Plant Family

Fabaceae (formerly called Leguminosae) is the third largest plant family in the world, with about 18,000 species. It is divided into three subfamilies, two of which have been discussed here earlier:

• the Mimosideae, which have flowers that look like soft spiny balls, with about 2,400 species and
• the Caesalpinioideae, which have brightly colored, showy flowers, with about 2,700 species, which includes the Senna and the Flamboyant trees.
• the Faboideae, the largest subfamily, with nearly 13,000 species.

It is this third subfamily to which the macano tree belongs. The flowers are Papilonaceous, which means they look like butterflies, but they are familiar to most of us as the distinctive flower of peas. These three subfamilies were organized originally mostly by the shape of the flowers, so let’s look at the shape of the pea flower to see why the macano belongs to the Faboideae subfamily with Papilonaceous flowers.

The Pea Flower

Here’s a diagram of a generic pea flower, copied from the Wildflowers and Weeds web site above (or next to, depending on your browser) an image of a flower from a macano tree. I don’t think there’s any doubt that we have a pea flower here.

What all Legumes have in Common

Regardless of their flower shape, all members of Fabaceae have at least the following three things in common, and many of them have the fourth thing as well:

1. compound leaves – explained below – often pinnate (like a feather)
2. pods – such as edible beans and peas
3. nitrogen fixation – particular bacteria living in special root nodules transform nitrogen from the air into usable fertilizer forms of nitrogen
4. moving body parts (most) – pods that snap open (some beans), leaves that fold on touch (mimosa), tendrils that “feel” for a new place, or reproductive parts that spring out of the keel of a pea flower when an insect enters.

The macano has all these features, although I can only illustrate the leaves and give a sense of how the reproductive parts are tightly coiled in the keel, enabling them to spring out.

First, the compound leaves.

Leaves are always attached to a stem. This may seem obvious until you look at an arrangement like this one from a macano tree. Here, small leaves are attached to a stalk, or axis, which in turn is attached to the stem. These small leaves, then, are not true leaves because they are not attached to the stem. They are leaflets, and the arrangement of leaf axis containing leaflets is called a compound leaf.

Tip: You can tell whether you have leaflets or leaves if you watch for the buds of new leaves. The bud always appears at the place where the leaf will form. The buds of the macano do not appear along the leaf axis, but appear along the stem. A bud for a simple leaf will give rise to one leaf. A bud for a compound leaf will give rise to a leaf axis with several leaflets on it. This is how we know that the macano has compound leaves, like the good legume family member that it is.

Now, for the contents of the keel – the moving body parts. I took a macano flower and tore it apart to look inside the keel. Here’s what I found:

I’m not sure I can describe the sensation of breaking open the keel. It had a longitudinal slit so it wasn’t hard to open, but when I separated the two sides, it felt like a minor explosion between my fingers. I didn’t know what had happened until I read later about the coiled spring for the reproductive parts. I wonder what insects experience when their mere presence causes the flower to burst open with an array of pollen-bearing parts!

The macano’s pods give the plant its scientific name, Diphysa americana. Di- meaning two; -physa meaning bladders in Greek. The sides of the outer layer of the pods have two inflated chambers. I haven’t seen the pods yet, but when I do, and get a picture, I’ll post it here.

As far as the nitrogen fixing parts of macano, well, I’ll have to take the scientists’ words for that. I think it’s neat, though, that where macano fence posts have sprouted, the soil will be enriched over time.

Diphysa americana

It was easy to find the scientific name of macano: both books Tropical Plants of Costa Rica (Zuchowski) and Trees and Shrubs of Panama (Carrasquilla) have good descriptions and pictures of the tree. Carrasquilla gives ‘macano‘ as a common name in Spanish. I was also able to find a pdf file on the internet that gives the complete botanical description of the species [Flores & Marin]. The following points come from that paper, with my own images to illustrate the points.

The species extends from Mexico to Panama, commonly on flatlands or moderate slopes in tropical dry florests at elevations of 5 to 800 m.

Its trunk is rarely straight.

The bark is brown or grayish brown, fissured, and rough.

We have already seen that the leaves are compound. They are also alternate – which means the leaf axes are not on opposite sides of the stem but climb up it in a staircase manner – and odd pinnate – which means there are an odd number of leaflets (between 11 and 21 for Diphysa americana), not an even number. The leaflets themselves are thin, soft, flexible, smooth, hairless, with toothless edges and rounded tips, shiny on top [left image] and almost grayish below [right image].

The image on the right, above, also shows the warty, longitudinal lenticels (areas on the surface of a twig). Click on the image for a larger view. The flowers occur in small, loose racemes, which are elongated clusters of flowers, shown at the beginning of this post, and for easy reference, below:

If you click on the image for a larger view, you’ll see that the green sepals that embrace the petals have a bell shape and you may be able to tell that there are 5 unequal lobes to this structure. The petals are deep yellow – pretty similar to the color shown by the camera. Inside the keel, as mentioned earlier, are the reproductive parts, the pollen-bearing stamens and the single female style.

The pod, when it appears, will be stalked, flattened, and pale brown. I suppose if you think you’ll be needing some ready cash in the distant future, and want to invest in macano wood, you might start looking for these pods in a few weeks and plant some seeds. Better yet, and perhaps faster, find a macano stick and plant it in the ground!