Lichens: Mysterious and Diverse by RICHARD E. WEAVER, JR. The science of lichenology, or the study of lichens, has lagged behind other branches of botany, and many aspects of lichen biology are still shrouded with mystery. In fact, the most mys- terious aspect of these plants, and the one basic to understand- ing them, was not known until the relatively late date of 1867: that although outwardly lichens appear to be discrete organisms, they are in fact made up of two very different kinds of plants bound together in a totally unique union. The components of lichens are these: (1) numerous individuals of an alga, usually of a one-celled green type similar to those which commonly give a green cast to the northern or shady sides of tree trunks, but occasionally a filamentous blue-green type like the ones which form the familiar blackish, rank-smelling scum on shal- low water, damp soil, or clay flower pots; and (2) strands of a fungus, similar and somewhat related to the bread molds. The arrangement by which they live together as a lichen is referred to as symbiosis, the close association of two dissimilar organisms with, in this case, mutual benefit. The exact nature of the lichen symbiosis, and the role that each of the components plays, is not completely understood. The fungus obviously provides protection for the alga, accumulates mineral nutrients, and helps in retaining moisture. The alga, because it contains chlorophyll, is able to synthesize carbohy- drates. There appear to be mutual exchanges of other organic nutrients, but these have not been identified. The balance between the components appears to be a pre- carious one, and slight environmental changes can in some cases upset it, resulting in the death of the lichen. But here an anomaly presents itself: lichens are able to prosper in some of the most extreme of the earth’s environments, in places where no other plants are able to survive. They are particularly abun- dant, for example, in parts of Antarctica. Because of their re- sistance to damage by extreme desiccation and cold, it has been suggested that lichens or lichen-like organisms would be able 133 134 to survive in extra-terrestrial environments. In fact, the pro- nounced seasonal color changes on the surface of the planet Mars have been suggested to be due to the presence of lichens. Several species have in fact been subjected to simulated Martian conditions, but none survived for more than a few days. In the remainder of this article I shall discuss various aspects of lichens which, I hope, will prove of interest to the reader. The final portion consists of a simplified, illustrated key to the iden- tification of some common species. Lichens and Air Pollution The possibility that lichens are intolerant of air pollution was suggested as early as 1866 from observations made around Paris, France. Since then, studies have been conducted in large urban areas on several continents, and the results are clear: the number of species as well as the number of individual lichen plants decreases as the center of a city is approached. The rea- son appears to be that the lichens accumulate toxic substances dissolved from the air of cities until they reach lethal propor- tions. The most important substances in the decline of urban lichens appear to be sulphur dioxide, a common component of the gases given off by the burning of fuels, and various fluorine compounds. The former primarily affects the algal component by destroying its ability to produce food through the process of photosynthesis. Lichens are decidedly rare in Boston, according to my per- sonal observations. Even in the Arnold Arboretum and Franklin Park, relatively large forested areas near the city limits, they are not common. The most common lichen in these places appears to be a species of Cladonia on the bases of trees. But even this plant rarely produces fruiting bodies. Various other Cladonias, several crustose species, and depauperate specimens of a Parmelia also are occasionally to be found. The forests in towns as close as Concord to the west and Walpole to the south support a reasonable diversity of lichen species. But the further one goes from any large city, the better the chances of finding a good development of lichen vegetation. Economic Uses When one considers the complete range of plant products utilized by man, those obtained from lichens are of minor im- portance ; yet lichens have proved to be of some use. The cell walls of the fungal component consist almost entirely of a unique starch called lichenin. As a result, lichens have some 135 food value, but the acids present render them somewhat un- pleasant to the taste. They have been eaten historically, but mostly in desperation. The manna of the Israelites, for ex- ample, was possibly a species of lichen. The Japanese, however, consider Umbilicaria species, or Rock Tripes, to be a delicacy. Cladonias and other fruticose genera of the far northern lati- tudes provide a forage crop for reindeer and caribou, and the native people of these areas harvest the lichens in large quan- tities to feed their semi-domesticated stock. But Arctic lichens apparently concentrate radioactive fallout from atmospheric atomic bomb tests, and the animals that eat them, and ulti- mately the people who eat them are showing signs of radiation poisoning. Various other animals, particularly invertebrates such as slugs and springtails (Fig. 1) also occasionally eat lichens. Besides the folk remedies concocted from various lichens, such as cures for lung disease from Lobaria pulmonaria and for rabies from Peltigera canina, lichens do have medicinal uses. Several of the lichen acids are effective as antibiotics, and com- mercial preparations are available in Europe. In addition, a number of species have had a major importance in the past as dyestuffs, and Harris tweeds still are made with the original lichen dyes. Figure l. Springtails feeding on a subfoliose lichen. 136 Structure and Form The plant body of a lichen is described as a thallus, a term referring to those plant bodies that are not differentiated into leaves, stems, and roots. A cross section through the thallus of most lichens would reveal a structure similar to that illus- trated in Figure 2. Both upper and lower surfaces of most lichens are covered with a tough protective covering known as the cortex. This tissue is composed of tightly packed fungal strands or hyphae. Immediately below the upper cortex is the Figure 2. The structure of a lichen in cross section, greatly simplified. algal layer, made up of loosely packed individual cells of the algal component interwoven with fungal hyphae. The medulla, with loosely packed fungal strands, makes up the bulk of the lichen thallus, perhaps as much as 2/3 of the thickness. Ex- tending from the lower cortex are hairlike appendages called rhizines, which anchor the lichen to its substrate much as roots do in higher plants. Unlike roots, however, rhizines do not ab- sorb water and mineral nutrients. There are numerous variations on the general pattern. For example, some lichens do not have a well-defined algal layer; rather, the algae are scattered throughout the medulla. Other lichens lack a lower cortex, and still others, lack rhizines. Lichens can generally be broken down into three easily rec- ognizable groups on the basis of growth habit (although there are intergrades between each of the groups). Crustose lichens (Fig. 3) are those that form an encrustation on their sub- strate, with such close contact as to be difficult to separate from it. Extreme forms actually have most of their tissues embedded in solid rock. Figure 3. The trunk of a red maple nearly covered with several genera of crustose lichens. The round black, gray, and pink (in life) structures are fruiting bodies. Figure 4. Two species of Parmelia, the largest genus of foliose lichens in our area, growing on a tree trunk. 138 Figure 5. Cladonia verticillata, a fruticose or "shrub- by" lichen, showing the upright podetia growing out of flake-like squamules. Figure 6. A species of Physcia with numerous, granular-appearing isidia. 139 Foliose lichens (Fig. 4) are perhaps the most familiar type. The thallus of these is organized into lobes or projections ra- diating from the center but maintaining a position more or less parallel with the substrate. Lichens of this type appear somewhat "leafy," hence the derivation of the term "foliose." Many species may be firmly attached to the substrate, but at least they can be pried off with a knife. A few genera of foliose lichens are more or less rounded in shape without distinct lobes. These are attached to the substrate only in the center of the thallus and are known as umbilicate lichens. Fruticose, or "shrubby" lichens are those in which the thal- lus is generally freely branched and stands away from the sub- strate. The most familiar genus of fruticose lichens, Cladonia, (Fig. 5) including those species commonly known as "British Soldiers" and "Reindeer Lichens," are unusual in that the plants are composed of two kinds of thalli: one, made up of flake-like, almost foliose segments known as squamules, growing close to the substrate; and the other, of upright structures known as podetia, which bear the fruiting bodies. In those species com- monly known as "Reindeer Lichens," the squamules disappear early in the development of the plant. In other familiar genera such as Usnea, the "Old Man’s Beard," the entire thallus is fruticose. Reproduction Many aspects of the reproduction of lichens remain a mys- tery. Most species appear to reproduce primarily by vegetative means, and indeed if any part of the thallus is broken off, it has the capability to produce a new plant.