Abstract
A study of the morphology
of certain lichen species gathered in the Quabbin Reservior area of central
Massachusetts was undertaken. Micrographs were taken with both light and
the scanning electron microscopes. Specimens were air dried and sputter
coated in preparation for SEM viewing and left intact for viewing with
the light microscope. SEM micrographs were obtained showing
algal and fungal structures surface structures, apothecia, and spores..
Introduction
Lichens are included with mushrooms, mosses, fungi, and ferns in a group of seedless plants known as cryptogams. They are in reality a symbiotic organism made up of fungi and algae, or cyanobacteria. Lichen symbiosis differs from other symbiotic relationships in that a new plant is formed that has no resemblance to it's former parts. The new thallus behaves as a single organism; the algae (photobiont) is responsible for photosynthesis, while the fungus (mycobiont) makes up the bulk of the plant body (Hale 1969). It is estimated that there some 20,000 different kinds of lichens and lichenologists agree that the number of identified lichens will only continue to increase.
Lichens can be counted among the slowest growing organisms on earth and as such are extremely sensitive to man made air pollutants. The progressive disappearance of lichens in European cities has been documented for over 100 years. Lichens exposed to a polluted atmosphere will die within a few months. Whenone considers that some lichens in the arctic regions are generally believed to be 1000 or more years old there is no such thing as quick recovery for a decimated colony. Since lichens have relatively less chlorophyll than other plants, even small amounts of sulphur dioxide (the main culprit) will destroy the balance between the alga and fungal symbionts. Thalli exposed to SO2 are thinner and discolored and the rate of photosynthesis is reduced. They also lack the ability to excrete toxic elements and this of course adds to their vulnerability (Hale 1967). Because lichens contribute to nitrogen fixation, serve as vital food sources in certain areas,are used to date geological formations, and have important antibiotic properties,their loss has an important impact on the environment.
Lichens are traditionally classified into three growth forms: crustose, foliose, and fruticose. These divisions represent differences in the structure of the thalli and range from primitive to highly structured. They are distinguished by the particular arrangement of cortical, algal, and medullary tissues and by different modes of attachment to the substrate (Hale 1983).
The simplest crustose lichens lack an organized thallus and consist of a hyphal mat that entraps algal colonies. On rocks, a hyphal mat and associated algae may grow among rock crystals and just under the rock surface contributing to chemical etching and eventual breakdown of the rock itself. Endolithic lichens lack any thalline growth on the surface of the rock it occupies. The vast majority of crustose lichens, grow on the surface of rocks and trees and have a distinct thallus. There is an upper cortex and an algal layer with a medulla of variable thickness which penetrates between rock layers or peridermal layers, firmly attaching the lichen to the substrate. The edge of the lichen may be easily discernible or indistinct from the substrate and a few groups have an alga free component distinguished by a blackened margin called a prothallus around the thallus. The highest stage in development of crustose lichens is the squamulous thallus. In this type the individual lobes, still lacking a lower cortex, become partially free of the substrate. This form is characteristic of many common soil lichens such as Catapyrenium, Cladonia, and Psora (Hale 1983).
Foliose lichens are leaf-like, flattened, and only partially attached to the substrate. Typically, they have distinct upper and lower surfaces distinguished by differences in color and surface structures. The thallus is usually divided into lobes that show varying degrees of branching and or overlapping. Laciniate lichens are the typical foliose lichens. They are lobate and vary in size. The lobes can be radially arranged or overlap like shingles on a roof. In some cases the thallus lobes can become inflated, having a hollow medullary center. The lower surface is often covered by rhizines, cilia, or atomentum. A tomentum differs from rhizines in that it's fibers are loosely organized strands and has a cottony texture. It is associated with forms that lack or havea poorly developed lower cortex. It is also common onthe upper surfaces of many species (Hale 1967).Umbilicate lichens have a circular thalli, consisting either ofone single, unbranched lobe or a multilobate thalli with limited branching patterns. All are attached to the substrate by a central umbilicus from the lower surface. The umbilicus usually consists oftightly packed, parallel arranged hyphae without photobiont cells. A remarkable group of foliose lichensare the so-called vagrant lichens. These lichens have evolved a unique protection from the desert conditions they live in. In the dry state, they roll up exposing their lower surface and are easily blown about in the wind. However, when they are able to take up water,either in the form of rain or dew, the thalli will unroll and expose the upper surface to sunlight (Nash1996).
Fruticose lichens are easily identified by their hair-like thallus that can range in length from 1or 2 mm to several meters. They may be strap-shaped or resemble shrubs and the lobes may be flat orcylindrical. The branching pattern of lobes varies considerably among different systematic groups and also within a single genus (Nash 1996). Like the other growth forms, fruticose lichens can be found in a wide range of climates and on various types of substrates.
The fungal component (mycobiont) of a lichen is an Ascomycete, Basidiomycete, or Deuteromycete. Asheterotrophs fungi have developed ingenious strategies for acquiring food; and, lichenization is a common and widespread method. A fungus undergoes lichenization when it acquires algal or cyanobacteria cells and begins it's symbiotic relationship. One out of five fungal species is lichenized (Nash 1969). Most lichen-forming fungi form non-stratified thalli (crustose, microfilamentous); 22% form either foliose or fruticose internally stratified thalli. Nearly forty genera of algae and cyanobacteria have been reported as photobionts in lichens. The most common photobionts are: Trebouxia, Trentepohlia, and Nostoc (Nash 1969).
As occurs in most fungi, the majority of lichenized ascomycetes have a
sexual and an asexual lifecycle. Within lichens usually only the mycobiont
expresses full sexual and to a certain degree, asexual reproduction. The
reproductive mode of the photobiontis reduced in the lichenized state.
In addition to the typical fruiting bodies of the individual symbionts,
lichens have evolved a number of vegetative structures by which both partners
are distributed (Nash 1996). It must be remembered that if
both partners are not distributed together they will not form a new lichen.
An apothecium is a fungal reproductive structure and while it produces
spores from which a new fungi may grow, it lacks the algal component. Soredia,
a cluster of algal cells wrapped in fungal filaments may disperse
and form new lichens.
Isidia are much like soredia except that it is enclosed within a layer
of protective cortex tissue; it is much more like a miniature lichen.<
Materials and Methods
Figure 2.
The scanning electron
microscope reveals a close up view of an apothecia from the above sample.
The bar represents 300 microns.
Figure 3.
Closer inspection
near the base of the above apothecia revealed this septate spore.
The bar represents 100 microns.
Figure 4.
Evernia mesomorpha
is a typical fruticose lichen. The thallus is a pale green and apothecia
are very rare. They are common on conifers, hardwoods, and even fenceposts
in Northern forests.This micrograph,taken with the light microscope shows
the substrate as well as the bushy nature of this lichen. The sample
is 17 mm in width.
Figure 5.
The scanning electron
microscope reveals the uneven branching and wrinkled surfaces. The
bar represents 5 mm.
Figure 6.
Closer inspection
shows the hyphal mat that keeps the lichen anchored to the substrate.
The bar represents 50 microns.
Figure 7.
Physcia americana
as taken with the light microscope is a typical foliose type. The
thallus is a pale green to whitish gray. The upper cortex is shiny
and moderately rhizinate. Apothecia are rare. They are common
in open deciduous forests in the Eastern half of the United States.
This sample is 21 mm in width.
Figure 8.
The scanning electron
microscope reveals the elegant surfaces of Physcia americana. This
view is magnified 11x.
Figure 9.
Higher magnification
shows a presumed granual which becomes the thallus forming areole.
The bar represents 25 microns.
Figure 10.
This micrograph shows
the typical internal structures of the thallus. The bar represents
300 microns.
A.) Cortex
B.) Symbiont layer C.) Medulla
Figure 11.
A closer look at fungal
structures. The bar represents 200 microns.
Figure 12.
7 septate spores cling
to the above structure. The bar represents 10 microns.
Discussion
Because lichens are so common in almost all corners of the world they are largely ignored by many. Their importance throughout history cannot be denied however. Their use by indigenous peoples for medicinal purposes, food, dyes, and even clothing is well documented. In mountainous and arctic regions, lichens are an important food source for animals. Because they are pioneer organisms they are important for soil stabilization and nitrogen fixation in recently cleared areas. The tenacious lichen will grow on nearly any surface because it does not depend on nutrients from it's substrate
The light microscope and the scanning electron microscope reveal the elegant details of the surface structures that are missed by the naked eye. In fact, the advent of scanning electron microscopy has greatly enhanced the understanding of this unique symbiont. There remains a need for more research to better understand thallus formation and propagation.
The lack of good field manuals to aid in identification is problematic.
The anticipated publication of The
North American Lichen Project in 2001 will be a welcome addition to
the available literature.
Bibliography