Thurber's stemsucker,
the parasitic pilostyles thurberi

The variety of previously unknown plants collected during the United States–Mexican Boundary Survey (1848–1855) often astonished the expedition’s botanists and naturalists. The variety in size alone was remarkable—from the enormous, awe-inspiring saguaro to a minute parasitic flower that confounded the naturalists because of its improbable growth habit. George Thurber, one of the expedition’s botanists, discovered this tiny flower, Pilostyles thurberi, growing directly out of the stems of Psorothamnus emoryi, an indigo bush, near the Gila River, in June 1850.

In a biographical account of Thurber’s life, H. H. Rusby noted: “It was upon this expedition that Dr. Thurber discovered the curious Pilostyles, which he at once recognized as something very curious, and which he subsequently accurately classified, insisting upon his determination in the face of the persistent incredulity of both [John] Torrey and [Asa] Gray, until his earnestness led them to an investigation and confirmation.”

Unlike other parasitic plants in our desert, Pilostyles thurberi (as the plant was named by Gray in Plantae Novae Thurberianae) is an endoparasite, living inside the host’s stems and invisible for most of the year. Only when it flowers does the plant push through the stems and become noticeable. What Thurber saw growing on the host indigo bush would have looked very much like an infestation of reddish scale insects clustered on the branches.

The cup-shaped maroon flowers are about 2 millimeters wide and are the only part of the plant that ever becomes visible, erupting through the host’s stems and branches in order to bloom. The cup holding the reproductive structures of the flower is formed by a ring of bracts that encloses two whorls of sepals. Flowers persist for a year or longer after maturity, eventually falling off the host and leaving behind a pitted surface on the stems.

The plant is shown here in an illustration from John Torrey’s Botany of the Boundary from the final Boundary Survey report. In the center of the drawing is a branch of Psorothamnus emoryi covered in the small round flowers of Pilostyles thurberi. Directly below this is a longitudinal section of a single stem (Fig. 1), greatly enlarged, showing the parasite’s flowers emerging from the host and the empty depression where a previous year’s flower had emerged. Fig. 2 is an enlarged single flower. A horizontal section (Fig. 3) is below the flower, and a longitudinal section (Fig. 4) is on the right, with both sections showing the ovules, one of which is enlarged in Fig. 5.

The name of the genus Pilostyles is taken from the Latin pilus, meaning “hair”, and stylus, or “pillar”. This “hairy pillar” refers to the reproductive structure at the center of the flower—a fleshy column topped by a disk that is present in both male and female flowers.

There is still some uncertainty about the plant’s pollinators and eventual seed dispersal and germination. Because the flowers are so inconspicuous, the plant relies on scent to attract pollinators. A 2014 study of Pilostyles thurberi on two species of Dalea in Texas found that its flowers emit a strong raspberry fragrance attractive to bees and wasps. Ants may also play a role in pollinating the flowers and in later dispersing the sticky seeds. Like our desert mistletoe, these sticky seeds may attach themselves directly to a stem and enter the plant’s tissues from there. Another possibility is that the seeds end up on the soil near the plant and invade the plant through the base of the main stem.

Pilostyles thurberi can be found in southeast California, southwest Arizona, southern Nevada, New Mexico, Texas, and Mexico. Currently there are nineteen species of Pilostyles occurring in the Americas and also in Africa, Iran, and Australia. The favored hosts for the entire genus Pilostyles are plants of the legume family.

Here are a number of photos of Pilostyles thurberi at various points in its development on Emory's indigo bush. — Cindy Hartwell

This article appeared in the May 2019 issue of the Desert Breeze, the monthly newsletter of the Tucson Cactus and Succulent Society.