Lichens

A student who was interested in natural dyeing came to me a number of years ago and asked if she could experiment with the dye potential of local plants as a special project. In the course of her experimentation, she obtained beautiful shades of yellow, brown, and green from two dozen common local plants, using simple "recipes."1 During the following summer, she extended the project to include lichens growing on the trees and rocks at a camp where she served as a counselor. The rich colors she obtained from the lichens were even more spectacular, which is perhaps not surprising since these organisms were, in the past, a major source of dyes and still are used in a minor way in commercial dyeing.2

Lichens traditionally have been referred to as prime examples of symbiotic relationships. Each consists of a fungus and an alga (or cyanobacterium) intimately associated in a spongy thallus. The thallus can range in diameter from less than 1 millimeter to more than 2 meters (0.04 inch to 6.5 feet). The photosynthetic component supplies the food for both organisms. The fungus protects the photosynthetic organisms from harmful light intensities, produces a substance that accelerates the rate of photosynthesis, and absorbs and retains water and minerals for both organisms. Supporting the belief that a lichen involves a symbiotic relationship is the fact that neither the fungus nor the photosyn-thetic organism grow independently where the lichen grows. The physiological evidence suggests, however, that it would probably be more correct to say that the fungus parasitizes the photosynthetic component in a controlled fashion, actually destroying chlorophyll-containing cells in some instances.

There are about 14,000 known species of lichens. The photosynthetic component is either a green alga or a cyanobacterium, a few lichens having two species of algae present. Three genera of green algae and one genus of cyanobacterium are involved in 90% of all lichen species, and one species of alga may be found in many different lichens. Each lichen, however, has its own unique species of fungus. With the exception of about 20 tropical species of lichens that have a club fungus and one species (associated with bald cypress trees) that has a bacterial component, lichens have members of the sac fungi for their fungal components. It is possible to isolate and culture the components separately. When this is done, however, the fungus takes on a very different, compact but indefinite shape, and the algae or cyanobacteria grow faster than they do when they are part of a lichen. The fungal component is very rarely found growing independently in nature, while the photosynthetic component is known to thrive independently of the lichen in

1. For further information on natural dyeing techniques, see Appendix 3.

2. Because the existence of many species of lichens is now threatened and they are also exceptionally slow growing, collecting of lichens for natural dyeing should no longer be encouraged.

Chapter 19

some instances. Lichen species, therefore, are identified according to the fungus present.

Lichens grow very slowly, at a maximum rate of 1 centimeter (0.4 inch) and a minimum of as little as 0.1 millimeter (0.004 inch) per year. They are capable of living to an age of 4,500 or more years and are tolerant of environmental conditions that kill most other forms of life. They are found on bare rocks in the blazing sun or bitter cold in deserts, in both arctic and antarctic regions, on trees, and just below the permanent snow line of high mountains where nothing else will grow. One species grows completely submerged on ocean rocks. They even attach themselves to manufactured substances, such as glass, concrete, and asbestos.

Part of the reason for the wide range of tolerance of lichens is the presence in the lichens' thalli of gelatinous substances that enable them to withstand periods of rapid drying alternating with wet spells. While they are dry, their water content may drop to as low as 2% of their dry weight, and the upper part of the thallus becomes opaque enough to exclude much of the light that falls on them. In this state, most environmental extremes do not affect them at all as they temporarily become dormant and do not carry on photosynthesis.

The lichen thallus usually consists of three or four layers of cells or hyphae (Fig. 19.35). At the surface is a protective layer constituting the upper cortex. The hyphae are so compressed they resemble parenchyma cells, and it is here that the gelatinous substances aid in the layer's protective function. Below the upper cortex is an algal layer in which algal (or cyanobacterial) cells are scattered among strands of hyphae. Next is a medulla consisting of loosely packed hyphae, which occupies at least half the volume of the thal-lus. A number of substances produced by the lichen are stored here. A fourth layer, called the lower cortex, is frequently but not always present. It resembles the upper cortex but is usually thinner and is often covered with anchoring strands of hyphae called rhizines.

Lichens have been loosely grouped into three major growth forms, which have no basis in their natural relationships but are convenient as a first step in their identification (Fig. 19.36). Crustose lichens are attached to or embedded in their substrate over their entire lower surface. They often form brightly colored crusty patches on bare rocks and tree bark. The hyphae of some that grow on sedimentary rocks penetrate as much as 1 centimeter (0.4 inch) into the rock. Others grow just beneath the cuticle of the leaves of tropical hardwood trees, with no apparent harm to the leaves. Foliose lichens have somewhat leaflike thalli, which often overlap one another. They are weakly attached to the substrate. The edges are frequently crinkly or divided into lobes. Fruticose lichens may resemble miniature upright shrubs, or they may hang down in festoons from branches. Their thalli, which are usually branched, are basically cylindrical in form and are attached at one point.

It should be stressed that while lichens may be attached to trees or other plants, the majority in no way parasitize them. There are, however, a very small number of species that do produce parasitic rhizines that penetrate the cortical parenchyma cells of their hosts.

Foliose Lichens

Figure 1Q.3S A section through a foliose lichen. (Photomicrograph by G.S. Ellmore)

upper cortex algal layer medulla lower cortex algal cells fungal hyphae rhizine

Figure 1Q.3S A section through a foliose lichen. (Photomicrograph by G.S. Ellmore)

Kingdom Fungi 377

Crustose Lichen PhotomicrographsReproduction Plant Through Runner

Although the fungal component of a lichen usually reproduces sexually, lichens are naturally dispersed in nature primarily by asexual means. In about a third of the species, small powdery clusters of hyphae and algae called soredia (singular: soredium) are formed and cut off from the thallus in a set pattern as it grows. Rain, wind, running water, and animals act as agents of dispersal. In other lichens, specialized parts of the thallus, known as isidia, may break off or be separated by decay.

Sexual reproduction in lichens is similar to that of the sac fungi except that the ascomata produce spores continuously for many years. No one has yet observed the initiation of a new thallus in nature, but it is believed that they arise after ascospores carried by the wind come in contact with independently living algae, germinate, and parasitize them. Lichen algae reproduce by mitosis and simple cell division.

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  • iole
    Is lichens a flowering plant?
    3 years ago
  • ROBERT
    Are lichens flowering plants?
    2 years ago
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    Is lichen a flowering plant?
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