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Manual de Plantas de Costa Rica

Main | Family List (MO) | Family List (INBio) | Cutting Edge
Draft Treatments | Guidelines | Checklist | Citing | Editors

The Cutting Edge

Volume XVI, Number 2, April 2009

News and Notes | Germane Literature | Season's Pick

Too often we take for granted plants that are right under our noses.  Growing literally underfoot, along the sidewalks of Santo Domingo de Heredia, in the administration/botany parking lot of INBio and throughout at least the Pacific portion of the Costa Rican Valle Central, is the bafflingly seldom-collected weedy herb Euphorbia prostrata Aiton (Euphorbiaceae).   Here we turn our gaze to this humble herb, and some of its kin among the chamaesyceous Euphorbia.  Known as Golondrinas (swallows, perhaps for the wing-shape of the paired leaves), these plants have a long history of medicinal use.

Left to right, these are E. prostrata (Hammel 25153), E. thymifolia L. (Hammel 25151), E. ophthalmica Pers. (Hammel 25150) and E. hirta L. (Hammel 25152)--ignoring, as we always do something, the grass!
Left to right, these are E. prostrata (Hammel 25153), E. thymifolia L. (Hammel 25151),
E. ophthalmica Pers. (Hammel 25150) and E. hirta L. (Hammel 25152) --ignoring,
as we always do something, the grass!

Having recently informed ourselves on the taxonomy of Costa Rican Euphorbia, via review of collaborator José González's Euphorbiaceae treatment for the Manual, we turned and tuned our eyes down to discover untold diversity; four species growing right here in the parking lot at INBio. The really odd and wonderful thing is that these very species pairs (E. prostrata vs. E. thymifolia and E. ophthalmica vs. E. hirta), the first of each pair rare in collections, the second very common and widespread, are precisely the pairs that are easily confused.   It turns out that much of what had been identified as E. (Chamaesyce) prostrata is actually E. thymifolia, and the humble E. prostrata is restricted to streets and sidewalks of the Central Valley area.

E. thymifolia, E. prostrata, E. thymifolia
E. thymifolia, E. prostrata, E. thymifolia

When growing together like this they are easily seen to be different entities, and on closer look they are vastly different.

Euphorbia prostrata
Euphorbia prostrata has fruits
(often not visible from above)
exerted from the cyathium and
pubescent in lines

Euphorbia thymifolia
Whereas in E. thymifolia the fruits
(easily visible right on top) are
± sessile inside the cyathium and
are pubescent overall

Euphorbia ophthalmica
Euphorbia ophthalmica and E. hirta

The other two species, side by side seem different, but just how different is not immediately obvious. At the level of cyathia and fruits, they are virtually identical, both having more or less pedunculate, dense heads of cyathia, and stipitate, pubescent fruits. It can take some time to convince oneself that E. ophthalmica is not just trodden and depauperate E. hirta.

Euphorbia hirta

Euphorbia ophthalmica

Euphorbia ophthalmica and E. hirta

The red midrib spot, usually present in E. ophthalmica is not definitive because it can also occur in E. hirta. The really constant differences between these two species, other than the ghestalt of leaf shape, color and reflective qualities, are that the former has only terminal inflorescences while the latter has them axillary and terminal.

Euphorbia ophthalmica
Euphorbia hirta
And at least in these populations the upper surface of
the leaves of E. ophthalmica has longer and
sparser trichomes than that of E. hirta

Of course E. hirta, given the opportunity, can become
much more splendid, even with ornamental
potential, than any of these others



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