PDF - CES (IISc)

INTRODUCTION
LICHENS are, with few exceptions, perennial aerial plants of somewhat
lowly organization. In the form of spreading encrustations, horizontal leafy
expansions, of upright strap-shaped fronds or of pendulous filaments, they
take possession of the tree-trunks, palings, walls, rocks or even soil that
afford them a suitable and stable foot-hold. The vegetative body, or thallus,
which may be extremely long-lived, is of varying colour, white, yellow,
brown, grey or black. The great majority of lichens are Ascolichens and
reproduction is by ascospores produced in open or closed fruits (apothecia
or perithecia) which often differ in colour from the thallus. There are a few
Hymenolichens which form basidiospores. Vegetative reproduction by
soredia is frequent.
Lichens abound everywhere, from the sea-shore to the tops of high
mountains, where indeed the covering of perpetual snow is the only barrier
to their advance ; but owing to their slow growth and long duration, they
are more seriously affected than are the higher plants by chemical or other
atmospheric impurities and they are killed out by the smoke of large towns:
only a few species are able to persist in somewhat depauperate form in or
near the great centres of population or of industry.
The distinguishing feature of lichens is their composite nature: they
consist of two distinct and dissimilar organisms, a fungus and an alga, which,
in the lichen thallus, are associated in some kind of symbiotic union, each
symbiont contributing in varying degree to the common support :
it is
a more or less unique and not unsuccessful venture in plant-life. The
algae Chlorophyceae or Myxophyceae that become lichen symbionts or
"gonidia" are of simple structure, and, in a free condition, are generally to
be found in or near localities that are also the customary habitats of lichens.
The fungus is the predominant partner in the alliance as it forms the fruiting
bodies. It belongs to the Ascomycetes 1
, except in a few tropical lichens
These two
(Hymenolichens), in which the fungus is a Basidiomycete.
types of plants (algae and fungi) belonging severally to many different
genera and species have developed in their associated life this new lichen
organism, different from themselves as well as from all other plants, not
only morphologically but physiologically. Thus there has arisen a distinct
class, with families, genera and species, which through all their varying forms
retain the characteristics peculiar to lichens.
1 E. Acton (1909) has described a primitive lichen Rotrydina vnlgaris, in which there is no
fruiting stage, and in which the fungus seems to show affinity with a Hyphomycete.