Friday, February 18, 2005
Star Rosette Lichen
"I find myself inspecting little granules as it were on the bark of trees — little shields or apothecia springing from a thallus — such is the mood of my mind — and I call it studying lichens." Henry David Thoreau
When and where seen: The star rosette lichen was found on a fallen branch in the Green Cemetery on February 6. I can't determine the type of tree other than it is deciduous. The star rosette is a fairly common lichen; it can be seen throughout the year and is probably all across town. Some was found last year on March 27, during a lichen foray in the Conant Land. I have some growing on the bark of a rock elm in the wooded part of my yard.
Word for the day: "Corticolous" means growing on bark. The star rosette lichen is nearly always corticolous. It is only very rarely found on bare wood or on rock. It favors the bark of poplars, alders and elms. Remember the smoky eye boulder lichen (Mosquito, November 12, 2004)? It is a saxicolous lichen i.e. it grows on rocks.
More lichen language: The body of a lichen is called a thallus and there are three common types of growth. The thallus of a crustose lichen, like the smoky eye boulder lichen, is tightly bonded to its substrate. Fruticose lichens, like British Soldiers (Mosquito, April 19, 2002) have an upright, shrubby form of growth with no distinct upper or lower surface. The star rosette lichen is an example of the third type, foliose, in which the thallus is flattened and somewhat leaf-like. Some foliose lichens are attached to their substrate by specialized structures called rhizines on the lower surface of the thallus. The color of the rhizines and the way they branch can be useful in identifying lichens.
Distinguishing characteristics: The thallus of the star rosette lichen is gray on top and white or pale brown on the underside. The lobes of the thallus are usually less than 2mm wide and as long as they do not encounter obstructions like other lichens, fungi or twigs, the growth radiates from a central point making a star-like pattern. There are many pale rhizines on the lower surface, and usually many dark disks called apothecia on the upper surface. Each apothecium has a ring of gray thallus tissue around it. All these characteristics can be seen with the naked eye, although a hand lens would help with the rhizines. The specimen in the photo is about one inch in diameter. Do not depend on the star shape for recognizing this lichen. 40B developments of cluster housing are common — you may find several feet along a branch completely covered with this and other lichens with no space in between.
Lichen reproduction: While a lichen is composed of both a fungus and a photobiont of some kind (usually an alga), it is only the fungal component that reproduces sexually. The fruiting bodies that produce the spores are either disk-shaped (called apothocia) or flask-shaped (called perithecia and pycnidia). When the fungal spore germinates, it may not be in a location that happens to have the right photobiont to recreate the lichen — so many lichens also use a variety of methods of vegetative reproduction. The absence, presence, placement, and form of the reproductive structures are vital to making an identification. The star rosette lichen has only apothecia and none of the specialized vegetative structures.
References: Lichens of North America, by Brodo, Sharnoff and Sharnoff; help from Elizabeth Kneiper, lichenologist, New England Wild Flower Society.
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