The following points highlight the two main examples of lichens. The examples are: 1. Parmelia 2. Usnea.
Example # 1. Parmelia:
It is a common example of a foliose lichen which is found growing on the bark as well as the rock intermixed with the mosses at higher altitude (above 4,000 ft.). The heteromerous thallus is lobed to deeply incised.
It is attached to the substratum by rhizinae which arise from its darker lower surface. Internally the thallus (Fig. 20.3) is differentiated into four tissues, namely, upper cortex, algal layer, medulla and lower cortex. The phycobiont is a green alga Pleurococcus.
The thallus reproduces vegetatively by fragmentation and also by isidia (Fig. 20.6) and soredia (Fig. 20.7). The spores are produced in typically cup-shaped fructifications called apothecia which may be sessile or shortly stalked.
The apothecia are scattered on the upper surface of the thallus lobes. They are large, round and have a thalline margin (Fig. 20.10). The asci are usually 8-spored. The ascospores are typically simple and colourless.
The paraphyses are septate and may be branched. They arise from the hypothecium which is colourless. Underneath the hypothecium are present the algal cells.
Example # 2. Usnea:
It is an example of a fruticose lichen with a cylindrical to ribbon-like, much branched thallus. It grows erect or pendant attached by a holdfast at the base only to the substratum (branches of trees). The thallus is usually grey-green and consists of a single main stalk or many.
The stalks arise from the base and fork repeatedly. The main stalks and the branches are covered with conspicuous, branchlet-like bristles or fibrils.
A cross section of the thallus reveals a central medullary tissue consisting of loosely arranged hyphae with many interspaces between. It is surrounded by an algal layer with a thin cortex external to it. The phycobiont is a green alga, Protococcus.
The thallus reproduces vegetatively by fragmentation. Some species (U. comosa) bear cigar- shaped soredia in whitish soralia.
The apothecia are large, plate-like and terminal in position. The apothecium has a thalline margin fringed with bristle-like outgrowths which may be simple or branched.
The disc of the apothecium is fawn in colour and about 1 cm in diameter. Each ascus contains 8 ascospores which are colourless and simple. Beneath the colouriess hypothecium is the algal layer.