FOLIOSE LICHENS

STUDY OF EXTERNAL FEATURES OF FOLIOSE LICHENS

Comments :-

• 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

Comments :-

• 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

Comments :-

• 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​

Comments :-

• 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​

Comments :-

• 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.