Trentepohlia aurea (Orange rock hair)

When I first encountered this on a rock face in Massachusetts, I was utterly perplexed. From a distance, I thought “lichen,” like elegant sunburst lichen or other bright orange lichens. (Sometimes it is classified as a lichen.) But up close, it was fuzzy, more like moss. Velvety. But not moss either—mosses are photosynthetic, hence green. A slime mold? Nope, doesn’t fit that modus operandi either. Not wet, not shiny. Soft.

Finally I identified it as algae. Algae? On land? Yup. Orange because it contains carotenoids, the same substances that color carrots.

Plants: Technically this is a filamentous terrestrial green alga. Yeah, not green, due to the carotenoids, but a member of the green algae family. It may be orange or yellow, and dulls if it dries out. The “hairs” are thalli, each up to 1/16″ (2 mm) tall, and branched or unbranched. Because it somestimes participates in the multi-species collaborative agreements known as lichens, it can also be part of a lichen. It colonizes rock faces, including natural rock and concrete, and sometimes colonizes humid, shaded barks, such as oaks and cherries.

Habitats: Prefers humid, shaded, alkaline habitats. It is most common in Europe and North and South America.

7/13/2024 · Amici's Restaurant, West Stockbridge, Massachusetts

Kingdom	Plantae	Plants, but not fungi, lichens, or algae (from Stearn's Botanical Latin)
Division	Chlorophyta	About 7000 species of green algae
Class	Chlorophyceae	Most green algae. From chloros, “green;” and phyceae, “algal organisation”
Order	Trentepohliales	Green algae that are subaerial (rather than aquatic), growing on humid soil, rocks, buildings, tree bark, leaves, stems, and fruit
Family	Trentepohliaceae	Algae with branching filaments without hairs, and both prostrate and upright growth systems
Genus	Trentepohlia	Algae that are orange rather than green due to carotenoid pigments that mask the chlorophyll
Species	aurea	Latin for “gold”

About plant names...