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PinkMonkey Online Study Guide-Biology
4. Lichens and mycorrhiza
Lichens
It is an obligatory association between an alga and a fungus, which together form a closely integrated unit.
The body of a lichen is composed of branching hyphae
of a fungus, which harbor algal cells. The fungus gets food synthesized
by the alga and the alga in return gets shelter, moisture and minerals
absorbed by the fugal partner from the substrate.
Lichens are classified into three principle groups as follows:
Figure 14.49 Types of lichens
(1) Crustose lichens : These lichens form a hard
granular crust closely adhering to rocks and tree trunks, e.g. Graphics.
(2) Foliose lichens : These lichens form leaf-like
thalli with lobed margins attached to tree trunks, rocks, walls, etc.
They are sometimes referred to as shelf fungus.
e.g. Parmelia.
(3) Fruticose lichens : These lichens form
much branched shrub-like habit developing fruiting bodies, e.g.
Usnea.
Economic importance of lichens : Lichens are used
in varieties of ways. Some lichens are valuable source of food to wild
animals like reindeer. Some lichens are fried and given to cattle as food
and to some extent to human beings. Some are used in medicines and other
are for preparation of dyes. Litmus is prepared from certain lichens and
some are also used in the preparations of cosmetics and perfumes.
Ecological significance : Lichens growing on rocks disintegrate them to form soil, preparing the ground for mosses and subsequently for higher plants. Thus, they help in the succession of plant communities.
Mycorrhiza
Symbiotic and usually non-pathogenic associations of
fungi with roots of higher plants, are called mycorrhiza. They are observed
in 90% of higher plants.
Figure 14.50 Mycorrhiza in Pinus root
The fungus decomposes the organic material in the soil,
thus making minerals available to plants, while in return they get sugars,
amino acids and other organic substances from the plant. It has been observed
that many forest plants die due to malnutrition when the soil lacks appropriate
fungi. The best known mycorrhizas are those of forest trees like Pinus
and orchids.
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