The Lichens
Page 370 to 372 of Chapter 18, Botany for Degree Students - Year I, BP Pandey

A lichen is a plant consisting of two separate plants – a fungus and an alga, so closely associated with each other as to appear a single plant. The algal cells of the association are always enveloped by the fungus. The combined growth of both parterns results in a constant definite form and internal structure of the lichen. This group has about 400 genera and 15 000 species and is treated separately instead of its connection with fungi or algae. The science of lichens is termed lichenology and one who studies this science is known as lichenologist.

The fungal component in four genera of lichens is a member of Basidiomycetes and in rest it is an Ascomycetous member as such the lichens are called Basidiolichens and Ascolichens. Basidiolichens have only four genera, all are restricted to tropical regions, while the Ascolichens are restricted to temperate regions. The algal components may belong to Myxophyceae (blue green algae) or Chlorophyceae (green algae). They may be filamentous or non-filamentous. Basidiolichens always possess a member of Myxophyceae. Some lichens have been observed, where fungi are associated with autotrophic bacteria. Each lichen is always formed of the same fungal and algal components and has constant internal structure and habit.

Three types of lichens: crustose, foliose, fruticose
1. Crustose
These lichens have flattened thalli, closely adherent to substratum. Thallus of majority of such lichens has more or less leathery texture and it internally differentiated with the algal component always restricted to a definite portion of the thallus. Some lichens have gelatinous thallus, in which alga and fungus are uniformly distributed throughout a gelatinous matrix. Lichens vary in form, colours and thickness. Thallus of these lichens may vary remain partly of fully submerged in the substratum on which they grow. In the case where body is fully embedded, only the fruiting body (ascocarps) of the fungus appear on the surface of the substrate.

2. Foliose
These lichens possess leave-like thalli with lobed or irregularly folded margins. Some parts of the thallus are more or less firmly attached to the substratum by means of hyphal outgrowths, rhizines from the lower surface. Rhizines may consist of separate single branched or unbranched hyphae, or of several parallel hyphae closely adhered to each other to form strands. A foliose lichen like Gyrophora may be attached to the substratum by a single rhizine growing from the centre of the thallus or a lichen may be attached by several rhizine.

3. Fruticose
These lichens have much branched, cylindrical, ribbon-like, flattened or sometimes filamentous thalli. They are much branched, and appear shrubby, and so the name fruticose (frutex = shrub) is given to them. They may be erect (Cladonia) or pendant (Usnea). Their thalli are attached to the substratum by the basal portion only which is composed of strands of densely packed hyphae.

The lichens are variously coloured commonly bluish-green or greyish-green. Many lichens are yellow, orange, reddish, brownish or black due to the presence of additional pigments.

Supplementary: Australian Lichen© 2012 Australian National Botanic Gardens and Australian National Herbarium, Canberra.

  • Fruticose lichens are erect or pendulous and markedly three-dimensional.
  • Crustose lichens are markedly two dimensional and firmly attached to the substrate. A crustose lichen looks very much like a thin crust on the substrate.
  • Foliose lichens could be thought of as halfway between crustose and fruticose. Though obviously three dimensional they grow in a more-or-less sheet-like form, but often with a lobed appearance. They are not attached by their entire lower surfaces to their substrates. Indeed, some foliose lichens are just centrally attached click to view to their substrates with the rest loose, so making it possible to see both the lower and upper surfaces very easily.
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