Finding Petrophytum & Sereno Watson's Ghost

Petrophytum caespitosum mat on limestone. Dark spots are shadows cast by flower clusters.

In a narrow rocky canyon on the east side of the Ruby Mountains, I was stopped in my tracks by Petrophytum caespitosum—rockmat. It's not that I was surprised. There's plenty of limestone in eastern Nevada. Rather I was pleased. It's always a treat to come across this striking rock dweller.

It was first collected by the great pioneering botanist Thomas Nuttall, probably in 1836, probably somewhere in the southeast quarter of Wyoming. His specimen—two small fragments, each with a flowering stalk—now resides at Harvard University (HUH; closeup below). Nuttall's yellowed handwritten label in the corner is terse: "Spiraea (Petrophyta. Caespitosa. Platte [River] sources." We shouldn't blame him. The expedition was on the move; time and space were limited. Large specimens and detailed location information were out of the question. Furthermore, it was an exploratory expedition. The country was still being figured out.

Spiraea caespitosa collected by Thomas Nuttall. (HUH; lower right corner of herbarium sheet; scale line added).

Nuttall's description of the new species was published in 1840. He named it Spiraea caespitosa, for its caespitose (matted) growth form. Most species in the genus Spiraea (meadowsweet) are erect shrubs, but this one was "A singular dwarf alpine plant (1), with scarcely the habit of Spiraea."  It was distinctive enough that Nuttall put it in its own section, which he called Petrophytum (rock plant). He also noted that "The taste of the plant is scarcely perceptible."

Sixty years later, Per Axel Rydberg of the New York Botanical Garden would argue that the rockmat was distinctive enough to warrant its own genus. He called it Petrophytum caespitosum, as it remains today (2) (Rydberg 1900).

The next known collection was made by William Whitman Bailey, botanist with Clarence King's Geological Exploration of the 40th Parallel. He found it in September of 1867, in the West Humboldt Mountains in western Nevada. His collection also resides at Harvard. But Bailey went further. That winter, he made a sketch of the rockmat eponymously sprawling across rock. It was added to the herbarium sheet, showing what can't be seen in a dried pressed specimen.

WW Bailey's Spiraea caespitosa (one of six specimens, from various collectors, on a single sheet; HUH).

Bailey's sketch of Spiraea caespitosa in the West Humboldt Mountains (HUH).

A year later, rockmat was again collected by a botanist with the King survey, this one an unlikely hire. Sereno Watson was in his early 40s, not a young man, with limited botanical training at most, and no previous expedition experience. But after a series of unsatisfying careers (teacher, medical student, insurance company secretary), he was desperate for change. In the spring of 1867, he sailed from New York to the promised land—California.

From San Francisco, Watson took the train as far east as it went, and then walked across the Sierra Nevada to King's survey camp. After presenting a letter of introduction from a mutual friend, he begged for a job. He would help in any way he could. King hired him as an assistant topographer and general camp helper, for "nominal" pay—basically a "volunteer" (Brewer 1900).

But the gods soon smiled on Sereno Watson. Bailey, the official botanist, began to have health problems, so Watson became his assistant. When Bailey quit in early 1868, Watson was appointed expedition botanist, with a salary. It was the beginning of a successful and very satisfying life collecting and studying plants (3). 

In early May of 1868, the expedition "took the field again and worked eastward from the Washoe through the Trinity, West Humboldt, Havallah, and the several other mountain ranges to Ruby River [possibly the Franklin River in the Ruby Valley], and from there the East Humboldt Mountains were explored" (Brewer 1900).

It was in the East Humboldt Mountains that Watson collected Petrophytum caespitosum. At that time, the East Humboldt range included the Ruby Mountains, which is where I saw it. Was the ghost of Sereno Watson nearby?

In his catalogue of collections, Watson provided a specific location: "Cliffs above Camp Ruby 7,000 ft." On my visit, I camped at the foot of the Ruby Mountains just 7.5 mi north of old Camp Ruby (also called Fort Ruby). I found the rockmat in a canyon immediately west. Sereno and I definitely were in the same area! And though I don't believe in ghosts, I do see them on occasion.

Petrophytum caespitosum at base of limestone cliff, east side Ruby Mountains; October 2021.

Caespitose rockmat is now known from many sites scattered across the interior western US and into Mexico. It seems to be restricted to limestone and limy sandstone. Though widespread and sometimes locally abundant, its distribution is curiously patchy. It's often absent from what looks like perfectly good habitat. Challenges in dispersal or establishment might be the explanation.

The growth form of Petrophytum caespitosum is striking. Its prostrate stems intertwine to form dense rock-hugging mats, sometimes a meter or more across. Short stems rise above the mat, with cylindrical flower clusters 1–4 cm long. The flowers are tiny, the white petals just 1.5–2.5 mm long.

Stems rise from clusters of leaves 3–12 mm long.

Inflorescences can be branched, but usually are simple cylindrical racemes.

As well as sprawling across rock, the rockmat can grow in vertical cracks, or even hanging from cracks!

Petrophytum caespitosum in a crack high up on the cliff (tip of arrow).

"The top of this two foot long plant is attached to the rock wall; the rest of the plant swings gently with any breeze." Photo ©Al Schneider, http://www.swcoloradowildflowers.com  

I end this story with a lovely shot by Andrey Zharkikh, who shares his many plant photos on Flickr. With the right setting, and a knowing eye, rockmat can be exceptionally photogenic!

Petrophytum caespitosum, with butterfly; Wasatch Range, Utah. Photo by Andrey Zharkikh.

NOTES

(1) In Nuttall's day, "alpine" did not necessarily mean the highest elevations, above treeline. It was sometimes used for lower montane sites with no trees—a common situation in the arid American West.

(2) The genus is sometimes called Petrophyton, said to be an "orthographic variant (misspelling)".

(3) Among other things, Sereno Watson became Curator of Gray Herbarium at Harvard in 1888, a position he held until his death in 1892 (Coulter 1892).

Sereno Watson, a "thorough and painstaking" botanist, working at Gray Herbarium (Coulter 1892).

SOURCES

Once again, I'm immensely grateful to the Biodiversity Heritage Library (BHL) for providing such easy access to original literature!

Brewer, WH 1903. Biographical memoir of Sereno Watson, 1820-1892. National Academy of Sciences. PDF 

Coulter, JM. 1892. Sereno Watson. Bot. Gaz. 17:137–141, Plates VI, VII. Available online courtesy BHL.

Nuttall, T. 1840. Spiraea cespitosa in Torrey, J, and Gray, A. Flora of North America v. 1, 417-418, Available online courtesy BHL.

Rydberg, PA. 1900. Catalogue of the flora of Montana and the Yellowstone National Park, vol. 1:206–207.  Available online courtesy BHL.

Watson, S. Catalogue of botanical collections made in Nevada and Utah, in 1867-9. Harvard University Botany Libraries. Available online courtesy BHL.

Williams, RL. 2003. A Region of Astonishing Beauty: The botanical exploration of the Rocky Mountains. Roberts Rinehart.