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.

Crystalline Beauty Amid the Garbage and the Flowers

On my recent geotrip in eastern Nevada, I often stopped at road cuts—those man-made features so beloved by geologists. A cut removes weathered rock, accumulated sediments, and those pesky plants. It often reveals rocks and structures below the surface. And when Fortuna smiles, a cut is in just the right place to expose something astonishing!

Of particular interest to me were road cuts in the Ruby, Schell Creek, and Snake ranges of northeastern Nevada. In these mountains there are rocks that have been around for a long time, and have suffered multiple episodes of deformation. Here's a condensed version of their history, to give a sense of what they've been through (DeCourten and Biggar 2017, DeCourten 2003).

Formation on a passive continental margin. Roughly 500 million years ago, much of eastern Nevada was a warm shallow ocean where sand, mud, and limey muck accumulated. With pressure, cementation, and enough time, the sediments became rock—thick layers of sandstone, shale, and limestone. This went on for hundreds of millions of years. Then Nevada's idyllic coastal setting came to an end.

Deformation caused by uplift and intrusion. By about 250 million years ago, the sea had disappeared. It was replaced with land pushed up and contorted due to the jostling of lithospheric plates far to the west. This also caused production and rise of magma, much of which cooled and hardened before reaching the surface. When magma was intruded into the old sedimentary rock layers, they were deformed and often metamorphosed (one reason there are so many productive mines in Nevada).

Deformation due to continental extension. Currently much of western North America is stretching, as it has been for maybe 30 or 40 million years. Nevada is about twice as wide as it was thirty million years ago! The old marine sedimentary rocks have been disturbed once again—uplifted and often tilted. As a result, mountains have risen and intervening land has sunk, forming the Basin and Range Province.

Northern Basin and Range Province (NPS). The many more-or-less parallel mountain ranges looked like "an army of caterpillars marching toward Mexico" to pioneering geologist Clarence Dutton.

In some of the areas I visited, extension and deformation have been extreme. Rock layers weren't just uplifted and tilted. Older rocks arching up from below broke the layers into huge chunks, which slid or were pushed for miles, greatly mangling the rocks (1). Geologists often say they read the rocks to understand the past. But rocks as mangled as these can be difficult to decipher.

With my limited background, I found this type of road cut cryptic, even with a guidebook. No matter, I poked around just the same. That's how I came upon this treasure amid the garbage and the flowers (2).

One of many beer cans for scale.

The closer I looked, the more beauty I saw. "Fractal" came to mind. Only after many photos did it concern me that I had no idea what this structure was, though somehow it reminded me of a geode. Turns out that's not far off.

The rock exposed here is mostly old limestone that formed when eastern Nevada was a shallow sea. Since then it has been greatly altered. Intruded magma probably played a role, for there is mineralization nearby (copper, silver, lead, zinc, and gold). The limestone also has been subjected to extreme extension, fracturing into huge chunks that moved for miles. In the process, the rock was broken into angular fragments, which were then cemented back into rock in various ways to form limestone breccia ("breccia" is Italian for rubble or rubbish).

Calcite veins in fractured limestone.

Calcite veins are common in this limestone, and sometimes broad cracks or cavities allowed the growth of larger crystals, which explains my treasure. This is the geode connection. Typically "geode" is used for a spherical or rounded chunk of rock containing a cavity lined with crystals. But other cavities qualify too, even caves, such as the Cave of the Crystals in Chihuahua, Mexico.

Gypsum crystals in the Naica cave, a giant geode. Alexander Van Driessche photo (source).

A geode may contain several kinds of crystals, and I wonder if my treasure is strictly calcite. It looks diverse to me, but according to Google, calcite can be clear, white, yellow, pink, purple, green, and more. Colors often are due to presence of metallic ions, referred to as chemical impurities. [Minerals and crystals are new topics for me. If you can add or fix something, please Comment.]

There were a few other things of interest that I recognized.

Fragments of limestone with quartz veins in a matrix of ... ??

Red coloration was common, due to iron oxide according to the guideboook.

Another puzzling but beautiful spot, with calcite, iron oxide, and an unknown rock.

And finally, there were flowers. Rabbitbrush, Ericameria nauseosa (many still call it Chrysothamnus nauseosus), was blooming along many roadsides.

NOTES

(1) This is a metamorphic core complex. I visited two on my trip, and may try to put together a post about them, but they intimidate me. It seems they're poorly understood, even by experts.

(2) From Leonard Cohen's Suzanne, a song that will always be with me.

SOURCES

DeCourten, F. 2003. The Broken Land; adventures in Great Basin geology.

DeCourten, F, and Biggar, N. 2017. Roadside Geology of Nevada.

Tingley, JV, and others. 2010. A Geologic and Natural History Tour Through Nevada and Arizona Along U.S. Highway 93. NV Bureau of Mines & Geology.

Tree Following & other news from the yard

I asked my young trees, "What does the future hold?" But all I heard was wind in the leaves.

From a distance, it looked like Flash the maple and Spike the hawthorn hadn't changed much in the two weeks I was gone. However, up close I could see the growing season is winding down. Maple leaves are ragged and dry, and hawthorn leaves are beginning to turn red and yellow. Snow and cold are in the forecast this week, but by next Sunday we're to be back to 60º F. I bet the hawthorn will keep some green leaves, and photosynthesize for awhile longer.

Flash, the Hot Wings Tartarian Maple, has been red for months.

Spike, the Winter King Hawthorn, gave life another try this year. Best wishes!

Hawthorn leaves are finally starting to turn.

Other News

Laramie had hard frosts while I was gone, and my native wildflower beds are mostly brown. But there are flashes of color here and there.

Bachelor's button and an unknown-to-me orange-rayed member of the daisy family are non-native annuals common in our native wildflower mixes. They bloom the first year after planting, and a few sometimes grow from seed after that, but not for long.

This little white-flowered member of the mustard family is new to me. It started blooming in the last few weeks. I also saw it in a rocky drainage on the prairie east of town a few days ago. I hope it survives long enough to produce fruit, which are required for identifying many mustards with confidence. Or do you know it?

Flowers are about 5 mm across.

Firewheel/blanketflower/gaillardia is the toughest of the wildflowers in the yard, always blooming longest and latest. Nice color, but it's looking a bit ragged.

Protected by a fence, this black-eyed susan is very much alive, with its dark disc flowers still opening, progressing from outside in.

This is my contribution to the October gathering of treefollowers, kindly hosted by The Squirrelbasket. Want to try it? More info here. By the way, it's easy and stress-free!