Gilding the Granite

A Guide to Lichens Found on Stone Walls

By Roger Monthey/Photographs by Stephen Sharnoff

The bright-yellow sulphur lichens on the [stone] walls of the Walden road look novel, as if I had not seen them for a long time. Do they not require cold as much as moisture to enliven them? What surprising forms and color! Designed on every natural surface of rock or tree. Even stones of smaller size which make the walls are so finished, and piled up for what use? How naturally they adorn our works of art! – Henry David Thoreau

Besides trees, nothing, perhaps, is more characteristic of woodlots in New England and New York than rock walls constructed by our forefathers. These structures add a sense of timelessness as we stroll through our woodlots, and cause us to wonder: who built them? why? how could they have done all this work? Tom Wessels, in Reading the Forested Landscape, writes that these rock walls were constructed to keep animals in or out of the open landscapes they enclosed. They were not used to mark boundaries; as Wessels points out, it is far easier to blasé trees than move rocks. Many of the boulders, if moved to their new rock wall locations, were rolled, pulled, pushed, levered by wood and iron poles, and otherwise randomly scraped and etched. This rough treatment removed some of the existing plant and fungal growth while creating new surfaces for pioneering organisms.

Over the years, these rock walls have developed a diverse and interesting assemblage of life forms, including mosses and liverworts, but they are a particularly hospitable home to lichens. Lichens consist of two organisms, an alga and a fungus, that work together symbiotically. The fungus attaches itself to a rock and provides shelter for the alga. The alga photosynthesizes food to keep itself and the fungus alive. Neither could survive there without the other, but together – as a lichen – they thrive. Lichens are capable of surviving in extreme conditions that would prove deadly to most other organisms, and are thought to be some of the oldest living beings in existence.

Several lichens species apparently prefer growing on undisturbed, natural boulders rather than on man-made walls, perhaps because of the rough treatment applied to some rock wall boulders. The smooth rock tripe (Umbilicaria mammulata) and the common toadskin (Lasallia papulosa) are examples. Some of the lichens found on rocks – hooded rosette lichen (Physcia adscendrens), for example –grow more abundantly on wood. The 13 species described below may serve as an introduction to the beauty and endless diversity of lichens readily found on the rock walls built by our forefathers.

Caloplaca flavovirescens (sulphur firedot lichen) – Species in the genus Caloplaca are called firedot or jewel lichens. The thallus, or plant body of most of these species, is bright orange or yellow-orange, as are the apothecia, the disc-shaped sexual reproductive structures. In the genus, apothecia, which serve to produce spores, usually have rims the same color as the thallus. Most species grow on rock, but a few are found on bark.   Caloplaca flavovirescens (sulphur firedot lichen)
Cetrelia chicitae (sea storm lichen) – The thallus is a greenish mineral gray, loosely attached, and 6-20 centimeters wide. This lichen has pores that are up to one millimeter in diameter and visible without a hand lens. The soredia, balls of algae wrapped in threads of fungus that serve as asexual reproductive packets, are very coarse and granular. There are no apothecia. The underside has a broad bare zone along the margins. It grows commonly on rocks and, more rarely, on trees in open forests.   Cetrelia chicitae (sea storm lichen).
Flavoparmelia baltimorensis (rock greenshield lichen) – The thallus is yellowish green and lies flat on or loosely attached to rock. It is 6-15 centimeters wide, though individual colonies can fuse together to cover large areas. The upper surface becomes wrinkly with age. It is covered with round, pustule-like growths that sometimes break down into granule-sized fragments. Apothecia are rare. The lower thallus surface is black with a narrow, bare, brown zone at the margins. This species is considered to be very common on rocks in open oak woods.   Flavoparmelia baltimorensis (rock greenshield lichen).
Lasallia papulosa (common toadskin) – The thallus is light brown, sometimes turning dull r red, 3-5 centimeters wide, and rather fragile. The upper surface is covered in blisters. The margins of the thallus become torn with age. Apothecia are very common, black, and smooth. The underside is brown to tan, deeply pitted, and bare. This species is very common on exposed boulders and ledges. Apparently, it is much less common on rock walls than on natural boulders unaffected by man.   Lasallia papulosa (common toadskin).
Pertusaria plittiana (rock wart lichen) – The thallus is gray, think or thick, and continuous. Modified apothecia are sunk into raised warts, which are part of the thallus. These warts are mostly flat on top with vertical or sloping sides, are 0.4-2.5 millimeters in diameter, and have one or more white-bordered pores. This species grows on rocks that are rich in silica, such as granite.   Pertusaria plittiana (rock wart lichen).
Physcia adscendens (hooded rosette lichen) – The thallus is pale gray and spotted with whitish round areas where gaps exist in the photosynthetic layer below. It forms small clusters of ascending lobes. Many of these lobes expand at the tip to form hollow, helmet-shaped groups of structures that work to produce vegetative reproductive packets, or soredia. This is a very distinctive feature. The hooded rosette lichen grows on bark, twigs, and – less frequently – on rock.   Physcia adscendens (hooded rosette lichen).
Physcia caesia (blue rosette lichen) – The thallus is whitish mineral gray, lies flatly attached to the rock, and is 4-9 centimeters wide. The lower surface is white to buff, and has a medium number (that is, not completely covered, nor completely bare) of rhizines, or root-like hairs that attach the thallus to the rock. Apothecia are rare. It is widespread on boulders and cliffs in fairly exposed areas.  
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Physcia phaea (black-eyed rosette lichen) – The thallus is whitish mineral gray, lies flat against the rock, and is 2-5 centimeters wide. The lower surface is white, with a medium number of Rhizines. Apothecia are numerous. It is common on sheltered rocks in open woods.   Physcia phaea (black-eyed rosette lichen).
Umbilicaria mammulata (smooth rock tripe) – The thallus is 3-25 centimeters wide and leathery, with a level upper surface. The underside has dense rhizines, and they are branched. Apothecia are rare. It is very common on large boulders in open woods. It is the commonest Umbilicaria in eastern North America. Apparently, it is much less common on rock walls than on natural boulders unaffected by man.   Umbilicaria mammulata (smooth rock tripe).
Xanthoparmelia cumberlandia (American rock shield) – The thallus is greenish yellow, forming circular colonies 3-12 centimeters wide. The central part tends to develop numerous lobes with age. Apothecia are common. The lower surface is tan and has a medium number of rhizines. It is common on exposed rocks.   Xanthoparmelia cumberlandia (American rock shield).
Xanthoparmelia plittii (rock shield) – The thallus is yellowish green, and is usually lying flat against the rock. It is 4-12 centimeters wide, and colonies of it often fuse to cover very large areas of rock. The lower surface is uniformly tan to light brown, with a medium number of rhizines. It is very common on exposed acidic rocks.   Xanthoparmelia plittii (rock shield).
Xanthoria elegans (elegant sunburst lichen) – The thallus is bright orange, lies flat on the rock, and is two to five centimeters wide. The lower surface is white, with sparse, coarse rhizines. Apothecia are very common and crowded. It is common on exposed cliffs and boulders.  
Xanthoria parietina (coastal sunburst lichen) – The thallus is deep to yellowish-orange, shiny, close or loosely attached, and 3-6 centimeters wide. The lobes of the thallus often overlap. Apothecia are numerous but not crowded. The lower surface is ivory-white with sparse, long rhizines. It grows near the seashore on trees and, more rarely, on rocks. It is similar to Xanthoria elegans, but when collected on seashore rocks, it can be differentiated by the broader, more distinctly leafy lobes and more numerous rhizines.  
Reformatted with permission for the internet May 2003 from the Northern Woodlands Spring '03 edition.