Biology Reference
In-Depth Information
Figure 3:
Some common cyanolichens. (A)
Peltigera leucophlebia
, on sandy soil. From northern interior British Columbia. (B)
Peltigera neopolydactyla
, on mossy dead
Picea
. From Montague Island, Prince William Sound, Alaska. (C)
Pseudocyphellaria
crocata
, on
Arctostaphylos
. From Oregon Dunes National Recreation Area, coastal Oregon. (D)
Nephroma arcticum
, on mossy
soil. From Southeast Alaska. Photographs courtesy Stephen Sharnoff, Missouri Botanical Garden, University of California,
Berkeley, CA, USA (http://www.sharnoffphotos.com/).
Color image of this figure appears in the color plate section at the end of the topic.
iv)
Internal structure
:
Structurally, the foliose type of lichen shows greater internal tissue organization.
A transverse section of foliose lichen reveals upper and lower cortex. Immediately beneath the upper
cortex there is an algal zone in which the cells of the photobiont are present. The distribution of
photobiont in the lichen thallus may be random through out the vegetative body of the thallus. Such
thalli are known as homoiomerous thalli. In majority of lichen thalli, the photobiont is distributed
just below the upper surface forming a well developed stratifi ed layer. Such thalli are known as
heteromerous thalli. The cells of photobiont are surrounded by thin-walled loosely packed hyphae
and at points of contact are shown to be intimately surrounded by the mycobiont. This is followed
by the central region known as medulla which is occupied by loosely entangled hyphal threads. The
medulla merges with the lower epidermis from which a number of rhizoids develop that anchor the
thallus to the substratum and also help in absorption. In case of crustose lichens the upper cortex is
composed of hyphal layer which is either rudimentary or highly developed. Beneath this layer an
algal layer is present in close association with the upper layer. A loosely entangled mass of hyphae
constitutes the medulla which then merges with the lower epidermis. The lower epimdermis may
be well developed or completely absent. In fruticose lichens there is no such differentiation as an
upper and lower surface but the thallus is composed of centrally grouped, compact hyphae that form
