Question

 LICHENS

 Lichens are actually two organisms, fungi and a photosynthetic microorganism, living together. Typically, lichens are comprised of cyanobacteria or green algae in a mutualistic relationship with an ascomycete fungus. The algae contribute food from photosynthesis and the fungus contributes anchorage and water retention. The lichen body, called a thallus, can be found on bare rock, soil, hanging from trees, and many harsh living environments. Lichens are generally the first organisms to colonize cleared rock and soil surfaces after volcanic activity and fires. 

Lichen growth generally appears in one of three forms. Crustose lichens are hard, flat forms that appear as crusts on rock or wood. Foliose lichens appear leaf-like. Fruticose lichens have erect, branching structures.

 Observe the various types of lichen growth forms.

Answer 1

Growth Forms

There are three main types of lichens:

1. Foliose
2. Fruticose
3. Crustose

1. Foliose Lichens

Foliose lichens have two easily distinguishable sides. In other words, there is a top side and there is a bottom side. They can be very flat, leafy like lettuce, or convoluted and full of ridges and bumps.

2. Fruticose Lichens

Fruticose lichens can be pendant and hair-like, upright and shrubby, or upright and cup-like. Many fruticose lichens have round branches that have a central core and others are hollow in the middle. Other fruticose lichens have flat branches that tangle up with each other.

3.Crustose Lichens

Crustose lichens are just that, crusts. They form a crust over a surface, like a boulder, the soil, a car, or your roof shingles. They can come in many bright, vibrant colors like sunny yellow, orange, and red, as well as grays and greens. Crustose lichens are pressed against their substrate.

Phylum: Mycophycophyta

Anatomy:

The anatomy of lichen thallus and fruiting bodies is examined under compound microscope with magnification 4 - 40x. The anatomy of the thallus is occasionally studied to see the thickness of various layers (upper cortex, algal layer, medulla, lower cortex), type of algae and their distribution (stratified – heteromerous or uniform – homeomerous) and to study the arrangement of fungal hyphae (vertical or horizontal) within the thallus (Fig. 26 (given above)) . The section of thallus can be cut with razor blade by keeping the fragment of thallus in potato or papaya pith. Microtome sections are very helpful but it is a time-consuming process. Just to check the type of algae present in the thallus one need not cut a section. By the colour of the thallus one can make out the type of alga (at least group) present. Lichen with blackish, bluish, slate grey thallus usually has blue green alga, while greyish, yellowish, brownish, greenish thallus has green alga. However, it is better to confirm the type of alga present by following an easy procedure. The algal layer of the lichen thallus is exposed by scraping the upper cortex with razor or snapper blade. Alga (which appears green, dark green, blue green, black) is picked up with blade or needle, transferred to the slide and examined. Sometimes in the field while differentiating lichens from fungi one can check for alga (green or dark green pigments) by exposing the upper cortex with blade.

Lichen thallus

Lichens grow in a wide range of shapes and forms (morphologies). The shape of a lichen is usually determined by the organization of the fungal filaments. The nonreproductive tissues, or vegetative body parts, is called the thallus. Lichens are grouped by thallus type, since the thallus is usually the most visually prominent part of the lichen. Thallus growth forms typically correspond to a few basic internal structure types. Common names for lichens often come from a growth form or color that is typical of a lichen genus.