A Neotropical Savanna

It still surprises me that there are so many members of the composite family (Asteraceae) in the tropics. It should not be surprising – the aster family and the orchid family are the two largest families of flowering plants. But somehow I feel the orchids belong in the tropics and the asters belong in the temperate world.

Even wikipedia says that although the aster or composite family is cosmopolitan, it is more common in the temperate climate and in tropical mountains. Nevertheless, the tropics are loaded with members of the aster family. Here in the western hemisphere, in the neotropics, we have 580 genera and 8,040 species of composites. By contrast, we have 300 genera of orchids and 15,000 species. (Statistics from Maas.) So, we have more species of orchids, but we have more genera of composites. Not as clear-cut as I previously thought.

Here in the savanna, in the dry season, the composites are conspicuous. We even have trees in the family Asteraceae – four of them are listed in the Trees Atlas of Panama.

One of those trees is the palo blanco – a brittle tree that suffered quite a bit of damage during the recent high wind episode.

Its scientific name is now Vernonanthura patens, although in its botanical history it has had at least six other names, one of which was Vernonia patens. The only complete description I’ve found for palo blanco is one in the Flora of Panama (Part IX. Family Compositeae), when it went under the name Vernonia patens. So I’ll be going through that description to show how to recognize the palo blanco, or Vernonanthura patens.

Vernonanthura patens is a Shrub or Small Tree to 8 m Tall

This young tree is no more than about 2 m tall, but I have seen them much taller, some on our own property, and one along the road to David that surely exceeded 8 meters – it seemed nearly 50 feet (15 m) tall.

The tree is “freely branched.” See how open and free the branches are in this young palo blanco. The branches themselves are marked with fine, usually parallel lines or grooves. You can see more freedom of branching in the image on the left, below, and the fine lines show up pretty well in the image on the right, especially when you click to enlarge it.

The younger stems and twigs may be covered with short, matted or tangled, soft wooly hairs.

Leaves with Short Stalks; Leaf Blades Lance-shaped

The leaf stalks or petioles are so short it’s hard to remove the leaf from the stem. The leaf blade is longer than it is wide, with the widest part below the middle (this shape is called lanceolate). It is sparsely covered with soft, short hairs on top and with fine, short hairs that may be stiff on the underside.

There may be 8 to 20 pairs of lateral veins.

Flowers Occur in Terminal Clusters, Much-branched, with Slightly Convex Head

The flower clusters, or inflorescences, are at the ends of the branches (terminal) (left image below), and just as the tree is freely branched, so the inflorescence is freely, and much, branched (right image below).

This type of inflorescence is called a corymb, which means that the lower stalks are longer than the upper stalks so the flowers form a flat or slightly convex head (American Heritage Dictionary). You can see the different sizes of the stalks in the next image, especially if you click on the image to enlarge it.

Flower Heads have 21-27 Florets

If you look at a sunflower closely, before the seeds form, you’ll see that the center of the flower consists of tiny individual flowers. Even the part of the sunflower that looks like a petal is another kind of flower. This grouping of tiny flowers into a “head” is what gave the aster family its older name, the composite family or Compositeae.

In the case of palo blanco, there are 21-27 small flowers in each flower head.

The Involucre is broadly Bell-shaped

An involucre is a whorl or rosette of bracts surrounding an inflorescence, or in this case, a head of flowers. (From the Latin involucrum, from involvere, ‘roll in, envelop’.) Involucres are the single most distinguishing feature or the aster family. They look like miniature artichokes. One of the first things I do now when looking at a flower cluster is to look for those multiple layers of bracts at the base of the flower. If they’re there – it’s a member of the aster family. If not – well, there are lots more families to choose from.

The bracts in the palo blanco come in a series of 5 or 6, closely overlapping like tiles on a roof, and are pale green with brown midrib or tip.

The Petals form a Slender Tube with 5 Lobes abruptly Spreading

If you enlarge the labeled flower head above and also the one immediately below, you’ll see several individual flowers with their 5 petals. The slender tube with 5 lobes abruptly spreading is called a “salverform” structure. 

You’ll also see several branched styles – part of the female reproductive system of the flower. These branched styles continue to develop until they take on a “ram’s horn” appearance, which is a characteristic of Vernonia-like flowers.

The male part of the flower’s reproductive system, the stamens, are hidden within the petals.

When you look closely at the enlarged images of the flowers, you’ll see tufts of hairs or bristles on the outside of the petals. These hairs are collectively called pappus and are another characteristic of the aster family. In the drawing of a Vernonia sp. from Delta-Inkey below, you can see the relation between the achene, which will become a small dry fruit with a single seed, the pappus, and the flower.

In the aster family, the pappus serves the same role as sepals in other flowering plant families.

Identification Summary

You can recognize the palo blanco, then, by

• it being a small shrub or tree to 8 m tall
• freely branched
• leaves with short petioles
• leaf blades lance-shaped
• terminal, corymb inflorescences
• 21-27 florets in flower head
• bell-shaped involucre
• 5 petals forming a slender tube
• branched styles

Distribution of Vernonanthura patens

This species is relatively common, extending from southern Mexico to tropical South America in the neotropics.

It is so common, in fact, that it is called a “weed” by the Germplasm Resources Information Network (GRIN). But as we all know, weeds are in the eyes of the beholder, and parts of palo blanco have been used to treat conjunctivitis, although without paying to see the entire article, I do not know which parts nor how effective it is. (Phytotherapy Research, Vol. 16, Issue 1, pp 1-22, 2002). Surely, also, its branches have been used as walking sticks or broom sticks – with a name like that.

What’s in a Name, Anyway?

The genus name , Vernonanthura, is named for 17th century English botanist, William Vernon. The species name, patens, means “spreading.” With “weed” still in mind, patens may refer to the ability of the entire plant to spread like a weed, but I’m guessing that the spreading refers to the tree form, which is rambling to say the least. The post began with an image of a nice, relatively compact young palo blanco. Here’s an older tree from our front yard.

Vernonanthura spreading.