FOLIOSE LICHENS

Contents

STUDY OF EXTERNAL FEATURES OF FOLIOSE LICHENS

The foliose lichens have a flat, leaf-like, lobed or deeply incised thallus.
It is attached to the substratum only at certain points by rhizines.
Rhizines are rhizoid-like outgrowths which arise from the under surface.
The thallus may be attached to the substratum either by a single rhizine or by several rhizines.
The thallus is generally greyish or brownish in colour.
Certain small, hard, dark and gall-like outgrowths called cephalodia may also be present. These help in retaining moisture.
The common examples include Parmelia, Physcia, etc.

These types of thalli are not very common.
Internal structure is not much differentiated.
The algal cells are irregularly scattered throughout the fungal hyphae.
Both algal cells and fungal hyphae are enveloped in a gelatinous matrix or the ground substance.

Internal structure of the thallus shows four distinct regions or zones. These are upper cortex, gonidial layer or algal layer, medulla and the lower cortex.
Upper cortex may or may not be bounded by an epidermis-like layer or hyphae.
Certain breathing pores may also be present in the epidermis. These help in gaseious exchange.
The upper cortex itself consists of vertical hyphae without intercellular spaces or with such spaces filled with gelatinous materials.
Beneath the upper cortex is algal or gonidial layer. It consists of numerous algal cells held together in the network of fungal hyphae.
Below the gonidial layer is the medulla made of very loosely interwoven hyphae.
On the lower side is the lower cortex, consisting of compact cells, lying either parallel or perpendicular to the lower surface.
Cells of the lower cortex produce rhizines.

Pycnidia or pycinidial cups are produced on the upper surface of the thallus.
Pycnidium is a flask-shaped cavity with a small opening at its apex, an ostiole.
The pycnidium is lined by the hyphae throughout its entire inner cavity.
Pycniospores are produced at the tip of these hyphae.
Pycniospores are released through an ostiole and germinate to produce a lichen if it comes in contact with an alga.

Apothecia are saucer-shaped fruiting bodies (e.g. Usnea, Physcia, etc.)
In some cases, perithecia, flask-shaped fruiting bodies are also formed.
Apothecium (also perithecium) is lined with palisade-like layer of cells called hymenium.
The hymenium consists of a series of elongated cells-the asci, intermixed with sterile hyphaethe paraphyses.
Each ascus usually contains eight ascospores but the number may vary from one to eight.
Ascospores, when liberated, if come in contact with the suitable alga, produce the lichen thallus.