98 



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 

 vegetative development in the lichen kingdom, since it secures the widest 

 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. 



