A Neotropical Savanna

This is the story of two plants that belong to the guava genus. The guava is a tropical fruit native to Latin America. [Illustration from wikimedia commons]. As far as I can tell, the two plants belong to the same species, but they differ in a couple of ways that are puzzling to me.

The first plant grows in very poor soil – even poorer than the average on our abandoned-pastureland/depleted-soil property. We have a few areas that are so rocky and the soil so thin that the only way it can be maintained is by weed-eater. On this particular slope we have let some shrubs survive the weed-eater in the hope that some day the area will develop into a natural site for native trees and shrubs.

Looking from the top of the slope down toward the gully where rainy season rain drains into the quebrada (seasonal spring) we can see several shrubs. I’ve circled a shrub of interest, coming closer to it in the next photo.

This shrub is in fruit now and although I failed to get a photograph when it was in flower last February, I have reason to believe it is a guayaba de sabana (guava of the savanna), Psidium guineense, which is in the Myrtaceae family.

The second plant grows in a relatively richer area. I did manage to get a photograph of one of its flowers, and you can see the white petals and many stamens of a typical myrtle family flower. The area has fewer rocks than the slope and can be easily mowed. At various other places on the property, we have similar shrubs, and all of them seem identical to this second plant. The plant on the slope seems different from the rest in ways that you’ll soon see.

What makes me think it’s a guava?
Almost as soon as I learned to recognize members of the myrtle (Myrtaceae) family, I knew that these plants were in that family. The flower above certainly fit. The vegetative characteristics, you may remember from other posts (on Eugenia and on Myrcia), are quite simple:

• simple, opposite leaves with smooth margins or edges – both plants have them (left=slope; right=meadow)

• punctations (translucent dots) on the leaves – the ones on the slope are harder to see in the images because the tiny hairs obscure the punctations, even after clicking to enlarge, but they’re there – the indentations at the end of the arrows on the meadow leaf are somewhat easier to detect, especially after clicking to enlarge (left=slope; right=meadow)

• thin, peeling bark (usually) – both plants have typical Myrtaceae bark (left=slope; right=meadow)

Determining the genus in this family, however, is not so simple. Gentry, who is so good at pointing out the simplest way to identify a family or genus, groups the various genera in Myrtaceae according to the features of the flower cluster, or inflorescence. Eugenia, for instance, is in a group where the

• inflorescence is a raceme (separate flowers attached by short equal stalks along a central stem)
• OR reduced to single flowers.

Psidium, by contrast, is in a group where the

• inflorescence usually has 3 to 15 or more flowers (with a shape different from a raceme botanically but that I don’t want to get bogged down by)
• OR may be reduced to single flowers on small stalks.

Gentry also points out the the leaves of Psidium have prominent secondary veins (the ones branching off the main vein running down the center of the leaf), a feature you can see on all the leaves in the images above. These features were not enough, though, to be certain of the genus. A Psidium specialist, Leslie R. Landrum, has stated that Psidium “…has long been one of the most difficult genera of American Myrtaceae to define. To do so, he uses a combination of floral and seed characters that I couldn’t possibly obtain in a short period of time.

So I turned to Carrasquilla, who has arranged the plants in his book, Trees and Shrubs of Panama, by family. He has only 5 plants listed in the family Myrtaceae, so it didn’t take long to recognize the genus – the guava genus called Psidium.

The Psidium genus

There are about 100 neotropical species of Psidium. They are all guavas and they are all evergreen trees or shrubs.

The name Psidium comes from the Greek word for pomegranate probably because guavas have lots of seeds, although guava seeds are certainly much smaller than pomegranate seeds, as you can see from the image of the ripe fruit at the beginning of this post and from this scan of the sliced green fruits from here.

Because most of the characteristics that distinguish Psidium from other genera in the Myrtaceae family have to do with fine details that I am not able to illustrate, I’ll jump right to the description of the species that Carrasquilla wrote about: Psidium guineense. The common name here in Panama is guayaba de sabana – the guava of the savanna – but it is also known as the Brazilian guava. The species name, guineense, was assigned by a botanist who believed that it originated from the Guinea Coast of Africa, but it did not. It is native to Latin America. It has been cultivated elsewhere, but you can see from this distribution map that its center is in the Neotropics. By clicking on the image you will see a few yellow dots in western Africa and northeastern Australia but all the rest of them are in Latin America.

Fruits of Warm Climates lists these characteristics, among others, for Psidium guineense:

• grayish leaves, 3.5-14 cm long and 2.5-8 cm wide
• leaves stiff, oblong, elliptic, ovate; scantily hairy on the upperside and coated beneath with rusty hairs
• flowers borne singly or in clusters of 3 in the leaf axils (where leaf meets stem) with 150-200 prominent stamens
• fruit round or pear-shaped from 1 to 2.5 cm

I highlighted the leaf characteristics because they 1) convinced me that the plant on the slope was P. guineense and 2) lead me on a bit of a goose-chase regarding the other plants on the property.

In the image below you can see that the undersides of the leaves (most of the leaves you’re seeing) have a rusty tinge. If you could feel them, you’d find them velvety with those rusty hairs. The leaves in the lower portion of the image are showing their upper sides and they do have that grayish tint to their green.

By contrast, the leaves of the plant in the meadow and of all other similar plants on the property are a greener color and have few hairs. If you were blindfolded and were handed a leaf from each plant, you’d have no trouble distinguishing the slope plant from the meadow plant.

Are there two species here?

For some time I was convinced that we had two guava species on the property. Not a single other plant on the property had the rusty-colored, velvety underside to the leaves that the slope plant has. Further, the leaves on the plant on the slope are smaller but the fruits larger than the leaves and fruits on the plant in the meadow. No other plant like this is on the property is in fruit, but the leaves of all the other plants have sizes like the plant in the meadow. I measured 10 leaves from each plant and got these averages:

slope leaves: 8 cm long x 4 cm wide
meadow leaves: 12 cm long x 7 cm wide

Fruits:

slope fruits: 3 cm long
meadow fruits: 1-2 cm long

I spent some time mulling over the differences, wondering whether they were meaningful, even reading up on the definition of species, which is disconcertingly vague. Then I remembered having read about the apple tree in Pollan’s The Botany of Desire: A Plant’s-Eye View of the World. The cultivated apple originated in the forests around Alma-Ata in Kazakhstan (Alma-Ata means “father of the apple,” says Pollan). A recent visitor to the area saw

“…entire forests of apples, three-hundred-year-old trees fifty feet tall and as big around as oaks, some of them bearing apples as large and red as modern cultivated varieties.”

So I thought, who knows how many species of apple were there in those forests? What’s important is that they were all apples.

And what we have here are guavas.

I did go so far as to track down descriptions of all six species of Psidium listed in the Flora of Panama Checklist, and I had to conclude that these plants are not different species. The main contender for the other species (which I really was rooting for) was Psidium guajava, the common guava. It, too, can have densely pubescent (hairy) undersides to the leaves. However, it has many more secondary veins (12-20 pairs) in its leaves than does P. guineense, which has about 8-10.

That clinched it. It seems to me that the number of veins in the leaves are much more likely to be characteristic of a species than the hairy-ness or the size. I content myself, then, with the guavas on our property, the guava of the savanna, Psidium guineense, while thinking of the apples in Kazakhstan, in all their variety.

Update: The online newspaper Hoy! from the Dominican Republic just published an article with recipes and health benefits of guava, an aspect of the fruit I completely failed to mention.