FOLIOSE LICHENS

FOLIOSE LICHENS

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.

 


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