STUDY OF EXTERNAL FEATURES OF CRUSTOSE LICHENS
- The crustose lichens are hard, granular crusts on rocks and bark of the trees.
- These adhere very closely and firmly to the substratum.
- The thalli are partially or completely embedded in the substratum.
- These lichens are generally ash-coloured. However, the colours may vary.
- The surface of the thallus is generally divided into polygonal areas called areolae.
- Some of the commonest examples include Lecidea, Graphis, Lecanora.
STUDY OF INTERNAL STRUCTURE OF CRUSTOSE LICHENS
- The thallus is poorly differentiated.
- The tissues forming the thallus are arranged more of less in strata, one above the other.
- The upper cortex is made of rudimentary or sometimes highly developed hyphal layer.
- Algal layer lies just beneath this layer. The layer shows agal filaments and the fungal hyphae in close association.
- Much below the algal layer lies the medulla composed of loose tissue of branching hyphae.
- The lower cortex lies next to medulla. It may be well developed or entirely absent.