Lichens

Lichens are a group of aberrant organisms formed by a symbiotic association of algae and fungi. The fungi component is called mycobiont and the alga component is called phycobiont. The two components are involved in an inseparable symbiotic association. The fungal component provides a structural cover and moisture while the algal component is involved in manufacturing food.

Lichens are normally found growing on tree barks, decaying wood, soil rich in humus, stones, rocks etc. Some are totally aquatic (E.g., Peltigera). Generally they are darkly pigmented.

Lichens vary in shape and size. Three main types are recognised based on habit and form:

Crustose Lichens

which have a flattened thallus, resembling crusts, attached to the substratum partially or completely.

E.g., Graphis, Verrucaria.

Foliose Lichens

which have a flat, expanded, leaf like thallus with rootlike rhizoids. E.g., Physcia, Gyrophora.

fig. 8.23 - Types of Lichens

Fruticose Lichens which have a thallus that is branched and bushy. It may be erect or pendant. Eg: Asnia, Evenia.

Lichens reproduce both asexually and sexually. Asexual reproduction normally occurs by fragmentation. In some forms, the algal component forms small outgrowths on the surface called isidia. These fall off and germinate into new thalli. Some lichens produce detachable structure called soredia, which can give rise to new thalli. Sexual reproduction involves the production of fruiting bodies called apothecia and perithecia produced by the algal component or ascospores and basidiospores produced by the fungal component.

Classification

Lichens are grouped into 3 classes based on the participating fungal component

1. ASCOLICHENS, in which the fungal component belongs to Ascomycetes.

2. BASIDIOLICHENS, in which the fungal component belongs to Basidiomycetes.

3. LICHENS IMPERFECTI, in which the fungal component belongs to Duteromycetes.

Mycorrhiza

Mycorrhizae are mutually beneficial or symbiotic association of fungi with roots of higher plants.

They are of two types:

Ectomycorrhizae

The bulk of the fungus grows over the surface as a mantle and only a part of it lives in the intercellular spaces of the root.

Endomycorrhizae

The fungus grows inside the cortex of the root, with some intra cellular hyphal tips. In some endomycorrhizae, the fungal hyphene develop some special organs called vesicles and arbuscules, within the root cortical cells such mycarrhizae are called vesicular arbuscular mycorrhizae (VAM). VAM are significant as they occur in a large number of crop plants.