Sole Native Pa. Plant Now Extinct

It has been 203 years since Curator Thomas Nuttall read his description of a new species of small subaquatic plant he discovered to the other members of the Academy of Natural Sciences of Philadelphia.

The new species was Hemianthus micranthemoides, and he discovered it along the Delaware River in Kensington, in Philadelphia. The species was later called Nuttall’s mudflower in his honor.

The mature plant was no more than 2 inches high with, Nuttall wrote, “Flowers white, scarcely a line long….” At the time, these flowers could be seen, albeit with some difficulty, from late August through September in wet places from Virginia to New York.

This specimen preserved in the Academy’s collection is the most recently collected Nuttall’s mudflower from Pennsylvania. It was collected in 1932.

Sadly, no one today is likely to witness the modest bloom of Nuttall’s mudflower, as it is the only plant species native to Pennsylvania that is now thought to be extinct. The demise of the last individual mudflower probably went unnoticed at the time. Even a botanist with a strong affinity for “ugly” plants would have to admit it lacks the charisma of a polar bear or panda.

But it is unlikely we are simply overlooking this species due to its unassuming appearance. No specimen has been collected for over 50 years, and all expeditions to locations where it was found previously failed to locate a single individual.

Original specimen used by Thomas Nuttall to describe the new species Hemianthus micranthemoides

An internet search for the name Hemianthus micranthemoides (or a later applied but equivalent name Micranthemum micranthemoides) would seem to provide some hope from the aquarium community, as numerous supply stores advertise the species under the common name “pearl weed” or “pearl grass.” Alas, the situation turns out to be an example of a taxonomic mixup.

Aquarium plants for sale that are misidentified as Micranthemum micranthemoides

The plant called “pearl weed” is in fact Hemianthus glomeratus, a related species native to Florida. This misidentification was confirmed when Academy Botany Curator Emeritus Ernie Schuyler ordered some pearl weed and compared it to the preserved material of Nuttall’s mudflower in the herbarium, including Nuttall’s original specimen collected in 1817 in Kensington. As far as we know, the only mudflowers left in the world are pressed on paper inside the metal cabinets of herbaria.

This rather tragic story highlights the importance and utility of natural history collections. Specimens of Nuttall’s mudflower provided information on historic collecting localities to serve as a starting point in looking for living populations of the plant as well as physical material for comparison with misidentified material from aquarium supply stores.

Nuttall’s illustrations of Hemianthus glomeratus published in the Journal of the Academy of Natural Sciences of Philadelphia in 1817

As wild habitat continues to be lost and species face the ongoing challenges of urbanization, climate change and invasive competitors/predators, collections in herbaria like the Academy’s will provide essential baseline data for conservation monitoring and planning.

Projects to digitize natural history collections are ongoing and provide a freely available resource to conservationists, land managers, policy makers and the general public. With the aid of these resources, perhaps we can honor the humble memory of Nuttall’s mudflower by preventing other botanical casualties in our Mid-Atlantic home.

For a list of ongoing projects and available databases in the Botany Department, visit our page on the Academy’s website.

By Jordan Teisher, PhD, Collection Manager, Botany Department

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In the Academy’s “Spotlight on the Collections” series, we tell stories about specimens chosen by our scientists and also how researchers and others around the world depend on our collections for issues involving climate change, water quality, evolution, and biodiversity and extinction.

To read previous installments in the “Spotlight on the Collections” series, visit:

A Fish Detective Story

I Am the Walrus

Down to Bond’s Bird in the Collection

The Real James Bond, Author

Insect Collection Valuable to Society

Gecko Collection: A Vital Resource

Protecting America’s Food Supply


  1. Has there been any comparison with that is labeled to be “dwarf baby tears”? In the hobby there is alot of back and forth between what is known as “dwarf baby tears/ pearl grass” which is typically labeled as Hemianthus glomeratus vs Hemianthus micranthemoides which is oven labeled as “pearl weed”.

    And what are the physical differences between the two and is does Hemianthus micranthemoides have a whorl pattern on three?

    1. Hi Sun,
      Thanks very much for your question. I am unfortunately not an expert on the genus Micranthemum (it seems all species in Hemianthus have been transferred to that genus), and the most recent systematic reference I could find in the literature (from 2013) suggested a thorough taxonomic revision of the genus is needed due to “weakly defined” species limits. Eileen Daub wrote a blog post in 2015 on That Fish Blog showing the difference in leaf shape between Hemianthus micranthemoides (actually H. glomeratus) and H. umbrosum. However, she refers to H. micranthemoides as “dwarf baby tears/pearl grasss”, while I have seen other sources refer the former common name to H. callitrichoides. It seems like the common names in this group of species are very inconsistent, and the scientific names have not been much better. Hopefully someone will tackle a revision and end all the confusion soon!

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