PDF - CES (IISc)

RADIATE THALLUS 101
E. prunastri, the second species of the genus, is more distinctly upright in
habit, with a penetrating basal hold-fast and upright strap-shaped branching
fronds, light-greyish green on the "upper" surface and white on the other
(Fig. 59). The internal structure is sub-radiate; both cortices are "decom-
posed"; the gonidial zone consists of somewhat loose groups of algae, very
constant below the "upper" surface, with an occasional group in the pith
near to the lower cortex in positions that are more exposed to light. There
is also a tendency for the gonidial zone to pass round the margin and spread
some way along the under side. The medulla is of loose arachnoid texture
and the whole plant is very limp when moist. It grows on trees, often in
dense clusters.
3.
FRUTICOSE AND FILAMENTOUS THALLUS
A. GENERAL STRUCTURE OF THALLUS
The conditions of strain and tension in the upright plant are entirely
different from those in the decumbent thallus, and to meet the new require-
ments, new adaptations of structure are provided either in the cortex or in
the medulla.
CORTICAL STRUCTURES. With the exception of the distinctly plectenchymatous
cortex, all the other types already described recur in fruticose
lichens; in various ways they have been modified to provide not only covering
but support to the fronds.
a. The fastigiate cortex. This reaches its highest development in
Roccella in which the branched hyphal tips, slightly clavate and thick-walled,
lie closely packed in palisade formation at right angles to the main axis
(Fig. 45). They afford not only bending power, but give great consistency
to the fronds. The cortex is further strengthened in R. fuciformis* by the
compact arrangement of the medullary hyphae that run parallel with the
surface, and among which occur single thick-walled filaments. The plant
and the narrow strap-
grows on maritime rocks in very exposed situations ;
shaped fronds, as stated above, may attain a length of 30 cm., though usually
they are from 10 to i8cm. in height. The same type of cortex, but less
highly differentiated, affords a certain amount of stiffness to the cylindrical
much weaker fronds of Thamnolia.
b. The fibrous cortex. This type is found in a number of lichens with
long filamentous hanging fronds. It consists of parallel hyphae, rarely septate
and rarely branched, but frequently anastomosing and with strongly thick-
ened "sclerotic" walls. Such a cortex is the only strengthening element in
Alectoria, and it affords great toughness and flexibility tc .the thong-like
1 Darbishire 1808.