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MORPHOLOGY
III. RADIATE THALLUS
i. CHARACTERS OF RADIATE THALLUS
In the stratose dorsiventral thallus, there is a widely extended growing
area situated round the free margins of the thallus. In the radiate thallus
of the fruticose or filamentous lichens, growth is confined to an apical region.
Attachment to the substratum is at one point only the base of the plant
thus securing the exposure of all sides equally to light. The cortex
surrounds the fronds, and the gonidia (mostly Protococcaceae) lie in a zone
or in groups between the cortex and the medulla. It is the highest type of
since it secures the widest
vegetative development in the lichen kingdom,
room for the gonidial layer, and the largest opportunity for photosynthesis.
Shrubby upright lichens consist mostly of strap-shaped fronds, either
simple or branched, which may be broadened to thin bands (Fig. 57) or
may be narrowed and thickened till they are almost cylindrical. The fronds
vary in length according to the species from a few millimetres upwards:
Fig. 57. Roccellafuciformis DC.