This Visitor Guide is courtesy of the Farm Park Preservation Association

Bizarre Organisms at Norristown Farm Park


This article originally appeared in the Fall 2018 issue of The Park Enthusiast.



You have probably noticed lichens on trees or rocks. One type that is common in this area looks like light-colored “medallions.” Another appears to be a mass of tiny green plants. Maybe you thought they might be mosses. But they are not. They are not even plants.

Lichens are complex organisms in a symbiotic partnership of fungi and either algae and/or cyanobacteria living among the fungal strands. The properties of the lichen differ from their component organisms.

Lichen Lichen

The fungi can be either an ascomycete (form spores in a sac called an ascus) in the left picture or a basidiomycete (from spores externally on the end of a specialized cell called a basidium) in the diagram on the right: The fungal component provides structure to the lichen as well as water and minerals obtained from the air or diffusion from the soil. Lichens do not have roots like plants do. The fungi also provide protection from the environment for the algae or cyanobacteria. The algae or cyanobacteria provide food via photosynthesis and, therefore, light is required. Cyanobacteria are sometimes called blue-green algae, but they are not related to eukaryotic algae. They are bacteria and very interesting in their own right. For instance, the chloroplasts in plant cells and the mitochondria in animal cells are symbiotic cyanobacteria that eons ago hitched a ride (endosymbiosis).

Most lichens attach loosely or strongly to just about any surface you can think of: tree trunks and branches, stones (some even grow inside a stone), roofs, hanging from branches, walls, gravestones, on soil as a soil crust. A few types float around not attached to anything. Lichens are found everywhere including deserts and the artic. They come in many different forms and colors. Most lichens grow very slowly (at an assumed regular rate) and they live for a very long time. These latter two characteristics can be used to date exposed rock surfaces based on the size of the lichen (lichenometry). The limit of this dating technique is 10,000 years and is very useful for dating for the period of 500 years or less, since radiocarbon dating is less accurate then.

Lichen forms

Lichens come in four major growth forms, but there are disagreements on just how many there are and some describe up to nine different forms. The crustoseform is tightly attached to the substrate and looks like a crust. Squamulose lichens look like closely clustered little pebbles. Folioselichens have flat, leaf-like lobes that are not tightly attached. Finally, the fruticose form looks like leafless branches that can be upright or hanging down. These forms are not rigidly defined and sometimes one form shares characteristics with another.

Lichen forms Lichen forms

From top to bottom in the diagrams, the thallus (body of the lichen) consists of a layer of tightly intertwined fungal fibers (hyphae) called the cortex, under which is the algal layer, where the algae cells are in close contact with more loosely intertwined hyphae. This allows for air circulation necessary for photosynthesis. The algal layer is part of the medulla (loosely intertwined hyphae). Sometimes this layer directly contacts the substrate (see Crustose diagram). In other lichens there is a lower cortex under the medulla which can form rhizines, root-like structures that function only in adherence of the thallus to the substrate.

Lichens can reproduce by asexually by fragmenting or by producing soredia, small groups of algal cells and fungal hyphae, in structures called soralia. The soredia are dispersed by the wind. Lichens can also produce sexually by producing spores formed after meiosis and fusion of gametes. Sexual reproduction is entirely a fungal function and the form it takes depends upon whether the fungus is an ascomycete or basidiomycete (see pictures above). The spores give rise to new fungi that then must find an algal or cyanobacteria partner to reform the lichen. This is not an efficient process.

Lichen forms Lichen forms

Lichens also come in many different colors which are determined by the photosynthetic partner (green for green algae and dark grey, brown or black for cyanobacteria) or by pigments resulting in red, orange, yellow or brown colors (for example, the yellow pigment usnic acid). The “paint” on the cliffs in the Rocky Mountains is due to lichens. Google “lichen images” and you’ll be amazed!

One last interesting fact: since lichens absorb everything in the air, they also absorb pollutants. If they absorb too many, they will die. This is why you may not see them near superhighways or in the middle of congested urban areas. However, at lower levels of pollutants, this absorption is used to monitor levels of different pollutants – so lichens are kind of like the canary in the coal mines.