Balsa

When a tree along the roadside comes into bloom, it always catches my eye, and I always think, “I have to come back and look at this more closely, and take pictures.” I don’t take my “photo-journalism” trips nearly as often as I’d like, but a couple of weeks ago I was able to study a tree that’s almost in our back yard – within a good hike of our house, anyway. I don’t know how many times I have passed this tree by without really seeing it, and then, there it was in bloom.

As soon as I saw those big stand-up flowers, I thought, “bat-pollinated.” It’s something a botanist friend taught me some time ago, and I’ve looked for it ever since. Bats are flying around at night, using radar, and they need a big, conspicuous object, lifted above the leaves, to zoom in on. A common, but non-native, bat-pollinated tree here in Panama is the African Tulip Tree, Spathodea campanulata. If you click on the following image to enlarge it, you’ll see why I could tell, even at a distance, that this tree is bat-pollinated, with those white vanilla ice cream cone flowers at the top.

This tree is the famed Balsa tree, the wood of which was used for the Kon Tiki, for airplane models and even for light wooden real airplanes. The word “balsa” is, according to wikipedia, Spanish for “raft.”

The scientific name for this tree is Ochroma pyramidale and it is in the Malvaceae family. Here’s why.

The Malvaceae Family

All the books I own, and many many internet sites, place the balsa tree in the family Bombacaceae. Who could resist a family whose name begins with Bombaca? It sounds like a dance, doesn’t it? Nevertheless, within the past few years, the Bombacaceae Family has been swallowed whole by the Malvaceae (mallow) Family. The mallow family contains interesting plants such as hibiscus, the marsh mallow, and cotton. The (formerly) “Bombaca” family contains such interesting plants as the baobab tree and the kapok tree.

From the similarities between kapok and cotton, you can see that the two (former) families were considered closely related, and indeed taxonomists were constantly putting some genera in one and then the other family. Finally, enough genetic studies were done to show that there was in fact no reason to keep the families separate, and so the Bombacaceae family, the smaller of the two, was placed within the Malvaceae. The situation is still a little bit in flux, but if you’re interested in these issues, there’s a good overview at the Malvaceae Info site and wikipedia shows exactly how the “bombacas” now fit into their new family.

I can find no explanation of how the genus Malva, which sounds somewhat malevolent to me, got applied to the innocent mallow plant, and thus to the entire family, so we’ll move right along to the important question of how you might recognize a member of the mallow family if you were to see it.

Mallows are recognized by these features (Elpel):

• flowers have 5 separate petals
• a distinctive column of stamens (they are fused together to make this column)
• alternate leaves, usually with indentations (lobes)
• mucilaginous leaves or other plant parts (okra, for instance, is in the mallow family)

Here’s how balsa fits these characteristics.

Five separate petals

The flowers in this tree were too high for me to reach, and by the time the flowers fell to the ground they were deteriorated. But you can see at least from the zoom on the camera that the petals are folded back on themselves and appear to be separate from each other. I believe we can see four in this image and can imagine that there’s a fifth behind it.

Better, here’s a screen shot from Tropicos, showing the flower after someone has opened it up to show the five separate petals and that “distinctive column of fused stamens,” which follows.

A distinctive column of fused stamens
 From the above image, you can get the idea that there’s something unusual about the center of these flowers. A dissected flower from Tree Atlas of Panama shows the stamen column at an early stage of development. The strange structure atop the column holds tiny pollen-laden stamens.

As the flower develops and the bats carry away the pollen (or it is simply shed), the structure unfolds to reveal the female organ, the white spiral pistil, shown in this drawing from Wikimedia Commons.

What we see in my image of the flower in the treetop, then, are the somewhat shredded pieces of stamen structure. The pistil is not visible from the ground.

Alternate leaves, usually with indentations (lobes)
 My images of the leaves on the tree make it hard to see whether the leaves are simple or compound, opposite or alternate, and you can’t even see whether there are lobes.

As is often the case, a drawing serves better to see the true features of these leaves. Again, the drawing is from Tree Atlas of Panama. In this drawing it is clear that the leaves are simple and alternate and that they have shallow indentations, or lobes.

Mucilaginous leaves or other plant parts
If you have a hibiscus or a hollyhock in your garden, take a leaf and crush it to feel what “mucilaginous” means. (For those in the Northern Hemisphere winter right now, apologies – you’ll need to defer this experiment until summer.)

In the case of balsa, it is the bark that exudes mucilaginous material.

Now that it’s clear that the balsa plant is in the Malvaceae Family, let’s move on to its genus and species.

Ochroma pyramidale
The scientific name of balsa is Ochroma pyramidale. The Ochroma part makes sense (ochros = pale yellow [Dictionary of Botanical Epithets]) because the flowers start out creamy white and turn yellow and then finally burgundy with age. But where did the species name pyramidale (pyramid-shaped) come from? What part of the plant looks like a pyramid?

The answer comes from a 1786 description of balsa when it was called Bombax pyramidale. “The fruit is a pyramidal capsule, 8 inches in length.” [Thank you Missouri Botanical Garden, for making this very old literature available online in a program called Botanicus, and thank you Babelfish for helping with translations from several different languages, in this case French, into English!] Modern descriptions of the fruit do not refer to it as a pyramid but rather as a long, ridged rod (Swartz), which perfectly describes its appearance in the tree at the time I saw it (illustration on the left, below). But another wonderful illustration in Tree Atlas of Panama shows how it can develop a point, giving the fruit, I suppose, the shape of a very long and narrow pyramid.

At any rate, Ochroma pyramidale is the only species of the genus in our neotropics. It is native from Mexico south through Ecuador and Peru as well as in the West Indies. It is also cultivated and naturalized elsewhere in the tropics (GRIN).

Seed pods
Although cottony seed pods were not listed as a unique Malvaceae characteristic, they do suggest a kinship between the cotton plant, the kapok tree, and balsa, as you can see in that second image, above, of the mature fruit. Here’s the wonderful description of Swartz on the process that gives the “cotton.”

When mature, the pods split lengthwise along several seams and allow the tightly packed cotton within them to force its way out. Expanding in the process, this soft material manages to cover the entire structure, coating it in tan-colored fluff. Thus opened, the mature fruit strongly resembles a large rabbit’s foot (hence its old scientific name: Ochroma “lagopus“). Embedded within the cotton are many, small 3-4 mm seeds. Fruiting occurs annually from mid-January through early April.

The small seeds are carried away with tufts of cotton by gusts of wind. After landing, they will not germinate unless the ground temperature is high enough to indicate the presence of direct sunlight (Swartz).

Recognizing the balsa tree
After having found my first balsa tree, I now see it in many places along the road to David. Those fruits sticking up in the air are a big signal. When I’m driving along the highway, it’s not that easy to stop and go through the list of characteristics that make this tree a member of the Malvaceae family, the

• 5 separate petals of the flowers
• distinctive column of stamens
• alternate leaves
• mucilaginous plant parts (in this case, rub the bark)

Instead, I can rely on those fruits, which will last until April. After that, what should I look for? Once again Tree Atlas of Panama provides help:

The large, heart-shaped leaves, weakly lobed, are distinctive. The other native species with similar leaves is the cuipo, Cavanillesia platanifolia, which is much less common along roads, has a straight trunk unbranched for most of its length, and generally has smaller leaves. Other species with heart-shaped leaves do not have the lobes (corners) of the balsa. See also the non-native teak, which has leaves quite like those of the balsa, and does occur pretty commonly along roadsides of the Canal area.

This fine description should keep us all from driving by the roadside balsa trees without really seeing them. Big, simple leaves. If there is any question about whether it is a balsa or a teak tree, stop the car, get out, and check whether the leaves are arranged on the stem opposite each other (teak) or are alternate (balsa). Once you know this, you know the tree, and you can drive on.