A Neotropical Savanna

Several of the living fence post species are in bloom now. Living fence posts are fence posts that have sprouted after becoming a part of a fence. They can be obvious posts with a few sprouts coming out the top, or they can have grown into full size trees while still serving as fence posts.

The yellow-flowered macano was a living fence tree described last year, and the “little machete” or machetito that we’re looking at today and that is shown below is in the same family – the Fabaceae or bean family.

The Flowers
Surely you can tell from the flower where the common name came from – the red petal looks like the blade of a machete and the calyx (the cup of sepals at the base of the petals) looks like the machete handle.

While looking at the flower, we can ask ourselves to which of the three large groups within the immense Fabaceae family this plant belongs. You may recall that the three groups, or subfamilies, are based on flowers that look like 1) peas, 2) mimosa (flowers like soft spiny balls), or 3) Caesalpinia-like flowers, which are brightly colored and showy flowers such as found in Flamboyant trees.

So which is this? Since the flowers are definitely not soft spiny balls, we are left with peas and with the showy flowers of the Caesalpinioideae subfamily. If you go to Wayne’s World, you’ll find a discussion of this group of trees, which are called in English “coral trees.” He has dissected one of these flowers here to show how this machete blade is really part of a pea flower, with all the remaining parts being reduced.

Let’s defer further thoughts on the flower for now – we will shortly look at them more closely to see just why just the one part of the pea flower should be conspicuous.

The plant loses its leaves – is deciduous – in the dry season, so the flowers are easily seen on the bare branches in this photo on the left taken in January. Yet in July when the leaves are out you can, with a judicious zoom (right), also see the flowers. Clearly, flowers can be present in both the dry season and the rainy season, but during the dry season they seem to be fuller and more abundant.

The Leaves
The leaves are compound – 3 leaflets for every leaf stalk or petiole. They are arranged on the stem in an alternate fashion, although some of the petioles are so close together on the stem that it is hard to see whether they are opposite or alternate in arrangement. Both these characteristics – compound leaves and alternately arranged leaves – are typical of the Fabaceae family.

The Tree
The tree can reach 10 m or 32 ft in height, which I believe this old one may have done – if it hasn’t exceeded it.

Sometimes I have to pause – a tree like this in the bean family?

Well, yes, because of those alternate, compound leaves, and because the fruits are, after all, beans, as you can see in this image of all parts from the Tree Atlas of Panama.

I learned the scientific name of the machetito from Carrasquilla‘s book Trees and Shrubs of Panama – it is Erythrina rubrinervia. But how did botanists know to place it in the Erythrina genus?

First, we know now that the flower puts it in the pea subfamily of the bean family (the Papilionoideae subfamily of the Fabaceae family).

Next, we know that it has compound leaves with 3 leaflets per leaf stalk. This makes it a trifoliate leaf. Within the pea subfamily, the only tree genus with trifoliate leaves is Erythrina (Gentry). So, that was easy. Erythrina it is.

Erythrina – red colored
The genus name, Erythrina, means red-colored. There are about 112 species in the tropics worldwide, and about 36 in Mexico and Central America (Neill, Ann. Mo Bot Gardens). The species extends into South America, and some time ago it was taken to Hawaii and cultivated there.

rubrinervia – red veined
The species name also contains reference to the color red, but what veins are they talking about? The leaf veins don’t look red to me.

After studying all the images at the Tree Atlas of Panama site, I found this close-up of the leaf stalk (petiole) which shows glands at the base of each of the leaflets, but which also shows red streaks on the petiole. They aren’t true veins but they give that appearance.

So we have a red-colored flower with red streaks on the petioles, giving us Erythrina rubrinervia. It’s incidental that the beans are also red.

Darwin’s interest
Darwin studied Erythrina species of plants, among others, when he was investigating how plants move. He found that the leaves of Erythrina and other species “sleep” at night, by drooping toward the plant stem. During the day the leaves orient themselves to be perpendicular to the rays of the sun in early morning and late afternoon and “edge-on” to the rays of the sun at midday (Project Gutenberg EBook). There’s a great sketch of the process at the web site of faculty member Thomas Herbert at the University of Miami.

Now let’s get back to that machete-shaped flower.

Hummingbirds
Whenever you see a long, narrow flower, you may suspect that hummingbirds pollinate these flowers, and this is true for the Erythrina genus as a whole, whose flowers are particularly rich in nectar. In our area, the larger hummingbirds – the Swallow-tailed Hummingbird and the Black-throated and Green-breasted Mango visit Erythrina (wikipedia).

These are long-billed hummingbirds. But if you should see a short-billed hummingbird at an Erythrina rubrinervia, you may yell “Thief! Thief!” because it has been shown that while long-billed hummingbirds pollinate the flowers, short-billed ones steal the nectar (Cotton, in Biotropica)

The Little Machete
So the little machete tree is filled with wonders – its bean-ness, its red-ness, the way it moves, and its relationship with hummingbirds. Watch for it next time you pass a living fence.