PDF - CES (IISc)

STRATOSE THALLUS 81
B. TISSUES OF SQUAMULOSE THALLUS
The anatomical structure of the squamules is in general somewhat
similar to that of the crustaceous thallus: an upper cortex, a gonidial zone,
and below that a medullary layer of loose hyphae with sometimes a lower
cortex.
1. The upper cortex, as in crustaceous lichens, is generally
of the
"decomposed" 1 or amorphous type: interlaced hyphae with thick gelatinous
walls. A more highly developed form is apparent in Parmeliella and
Pannaria where the upper cortex is formed of plectenchyma, while in the
squamules of Heppia the whole structure is built up of plectenchyma, with
the exception of a narrow band of loose hyphae in the central pith.
2. The gonidia are Myxophyceae or Chlorophyceae ; the squamules in
some instances may be homoiomerous as in Lepidocollema, but generally
they belong to the heteromerous series, with the gonidia in a circumscribed
zone, and either continuous or in groups. Friedrich 2 held that, as in crustaceous
lichens the development of the gonidial as compared with the other
tissues depended on the substratum. The squamules of Pannaria micro-
phylla on sandstone were lOOyu- thick, and the gonidial layer occupied 80 or
that may be compared Placodium Garovagli on
90 /A of the whole 3 . With
lime-containing rock: the gonidial layer measured only 50 //. across, the
pith hyphae 280 p and the rhizoidal hyphae that penetrated the rock 500 //..
3.
The medullary layer, as a rule, is of closely compacted hyphae which
give solidity to the squamules; in those of Heppia it is almost entirely
formed of plectenchyma.
4. The lower cortex is frequently little developed or absent, especially
when the squamules are closely applied to the support as in some species
of Dennatocarpon. In some of the squamulose Lecanorae (L. crassa and
L. saxicoUi) the lowest hyphae are somewhat more closely interwoven;
they become brown in colour, and the lichen is attached to the substratum
by rhizoid-like branches. In Lecanora lentigera there is a layer of parallel
is reached when
hyphae along the under surface. Further development
a plectenchyma of thick-walled cells is formed both above and below, as in
Psoroma hypnorum, though on the under surface the continuity is often
broken. The squamules of Cladoniae are described under the radiate-stratose
series.
1 See p. 83.
- Friedrich 1906.
3 See p. 76.