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MORPHOLOGY
II. STRATOSE THALLUS
i. CRUSTACEOUS LICHENS
A. GENERAL STRUCTURE
In the series "Stratosae," the plant is dorsiventral, the tissues forming
the thallus being arranged more or less regularly in strata one above the
other (Fig. 37). On the upper surface there is a hyphal layer constituting
Fig. 37. Vertical section of crustaceous lichen (Lecanora subfusca
var. chlarona Hue) on bark, a, lichen cortex; b, gonidia;
c, cells of the periderm.
x 100.
a cortex, either rudimentary or highly elaborated ; beneath the cortex is .
situated the gonidial zone composed of algae and hyphae in close association
; and deeper down the medulla, generally a loose tissue of branching
hyphae. The lower cortex which abuts on the medulla be absent.
may be as fully
developed as the upper or it may
The growing tissue is chiefly marginal ; the hyphae on the outer edge
remain "meristematic" 1 and provide for horizontal as well as vertical ex-
tension; and there is also continual increase of the algal cells. There is in
addition a certain amount of intercalary growth due to the activity of the
gonidial tissue, both algal and fungal, providing for the renewal of the
cortex, and even interposing new tissue.
B. SAXICOLOUS LICHENS
a. EPILITHIC LICHENS, The crustaceous lichens forming this group
spread over the rock surfaces. The support must be stable to allow the
necessary time for the slowly developing organism, and therefore rocks that
are friable or subject to continual weathering are bare of lichens.
aa. Hypothallus or Prothallus. The first stage of growth in the lichen
thallus can be most easily traced in epilithic crustaceous species, especially
in those that inhabit a smooth rock surface. The spore, on germination,
produces a delicate branching septate mycelium which radiates on all sides,
as was so well observed and recorded by Tulasne 2 in Verrucaria muralis
(Fig. 14). Zukal 3 has called this first beginning the prothallus. In time the
1 Wainio has adopted this term for growing hyphae 1897, p. 33.
2 Tulasne 1852. 3 Zukal lg
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