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.
STUDY OF INTERNAL STRUCTURE OF FOLIOSE LICHENS
- 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.
STUDY OF INTERNAL STRUCTURE OF HETEROMEROUS THALLUS
- 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.
STUDY OF PYCNIOSPORES
- 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.
STUDY OF ASCOSPORES
- 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.