Buellia pullata 1

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Identification Of Two Black Crustose Lichens
In The Palomar College Arboretum

1. Buellia pullata Tuck.

Buellia Family (Buelliaceae)

For many decades I have pondered over the identity of two black, crustose lichens covering granitic rocks and boulders in the Palomar College Arboretum. I decided to see if I had sufficient brain power to key these lichens down to species. I used the following two classic references and then verified the name with the University of California Checklist of California Lichens:

  1. Fink, B. 1935. The Lichen Flora of the United States. University of Michigan Press. 424 p.

  2. Hasse, H.E. 1913. "The Lichen Flora of California." Contributions From the United States National Herbarium 17 (1): 1-132.

  3. University of California Checklist Of California Lichens by Shirley C. Tucker & Bruce D. Ryan

Dark ascolichen (Buellia pullata) on rocks & boulders in the Palomar College Arboretum.

Dark ascolichen (Buellia pullata) on rocks and boulders in the Palomar College Arboretum.

Unlike other crustose ascolichens in the area, this species does not have conspicuous apothecia. In fact, the apothecia are very difficult to spot and are barely visible with the naked eye. The thallus is very thin compared with pyrenocarpous lichens of the genera Verrucaria and Staurothele. From a distance, it appears like a thin coating of black paint. Under close examination it resembles a thin layer of dried, cracked mud. Microscopic examination of the thallus reveals scattered black apothecia. Under 20x magnification, they appear like minute spherical (disk-shaped) buttons about 0.2 to 0.5 mm in diameter.

Magnified view of the thallus of Buellia pullata showing a minute apothecium (red arrow) compared with the head of an ordinary straight pin. The apothecium is about 0.5 mm in diameter. The pin head is approximately 1.5 mm in diameter. 20x magnification under dissecting microscope.

 Size Of Straight Pin Used In Wayne's Word Articles 

Magnified view of the thallus of Buellia pullata showing a minute apothecium (red arrow). The apothecium is about 0.5 mm in diameter, roughly the length of a single Wolffia globosa. Compare this dimension with an average cuboidal grain of table salt (NaCl) which is approximately 0.3 mm on a side. The thallus is divided into minute sections (areoles), like dried, cracked mud. 40x magnification under dissecting microscope.

 Size Of Salt Grain Used In Wayne's Word Articles 

When a water-soaked fragment of the apothecium is placed on a glass slide and mashed with a cover slip, the true identity of this lichen is revealed. This requires careful examination under high power (400x) with a compound microscope. Each ascus contains eight brown, 2-celled ascospores. The asci are surrounded by paraphyses (hairs) that have black, club-shaped tips. The oval spores are about 12-16 µm long.

Magnified view of the apothecium of Buellia pullata. A. Brown, 2-celled (bilocular) ascospore. B. Ascus containing eight ascospores. C. Immature ascus. D. Black-tipped paraphyses (hairs). The length of each oval ascospore is 12-16 µm about twice the diameter of a single human red blood cell (erythrocyte). 400x under compound microscope.

 Table Of Cell Size Comparisons 

Technical Description: Thallus granular to minutely rimose-areolate, dark brown or black; apothecia adnate, minute (0.2-0.5 mm), flattened to slightly convex; margin of disk lecideine and non-thalline (with no photobiont cells), only the proper exciple present (inconspicuous and same color as disk); paraphyses club-shaped at apex and black-tipped; spores oval, eight per ascus, brown, bilocular, 12-16 µm in length.


2. Verrucaria nigrescens Pers.

Verrucaria Family (Verrucariaceae)

Another dark brown (black), crustose rock lichen in the Palomar College Arboretum. The thallus is areolate (cracked into sections called areoles) like dried, cracked mud. It is thicker than Buellia pullata and the ascocarps are perithecia rather than apothecia. The orange lichen is Caloplaca bolacina. The thallus is composed of small scalelike, lobed (lobulate) squamules. The squamules are similar to areoles except they lift slightly from the rock surface along the margins. The apothecia are larger and more conspicuous than Buellia pullata, with an orange disk and paler thalline rim (margin). Since the rim is the same color as the thallus and contains photobiont cells, it called lecanorine.

Another crustose rock lichen with a dark brown or black thallus grows on rocks and boulders in the Palomar College Arboretum. Unlike Buellia pullata, this is a pyrenocarpous lichen with perithecia rather than apothecia. The perithecia are embedded in the thallus and are very small, slighter smaller than the apothecia of B. pullata. Younger perithecia are almost completely embedded in the thallus. In older perithecia, the emergent portion of the perithecium is conical-spherical, 0.2-0.4 mm in diameter. The spores are colorless and simple (non-septate), about 20 µm long and 8 per ascus. The paraphyses break down or gelatinize (deliquesce) and are not visible when the spores are mature. The thallus is areolate, cracked into irregular sections (areoles) like dried, cracked mud. Another species in San Diego County, V. viridula, is very similar except the cracks between the areoles are wider. In the latter species the areoles are more distinctly separated from each other. In addition, the thallus of V. viridula appears greenish-brown when wet. V. nigrescens has an enormous worldwide distribution, including North America, Europe, Asia and northern Africa. Aquatic species of Verrucaria grow on boulders along streams and waterfalls, and the marine V. maura grows on rocks of the wave battered intertidal zone.

Magnified view of Verrucaria nigrescens. Younger perithecia are almost completely embedded in the thallus. The cracks in thallus are thinner than V. viridula. In addition, V. viridula has more widely separated and distinct areoles. When wet, the thallus of V. nigrescens does not appear greenish as in V. viridula. 20x under dissecting microscope.

Magnified view of an ascus of Verrucaria nigrescens. The colorless, simple (nonseptate) spores are about 16-22 µm in length. 1000x under compound microscope.

 See The Marine Lichen Verrucaria maura 
Ascus & Spores Of Verrucaria viridula