TAXONOMY 
229 
SUBDIVISION V— LICHENES, THE LICHENS 
Lichens are variously colored, usually dry and leathery plants, 
consisting of symbioses of algae and fungi. In each case the fungus 
derives its food from materials manufactured by the algae and in 
return extracts water from the substratum and shares it with the 
algae. The association is therefore mutually beneficial. Blue- 
green and Protococcus forms of Green Algae and Ascomycete Fungi 
are for the most part concerned in lichen formation. 
Fig. 105. — A'foliaceous lichen, Physcia slellaris (L.) Nyb., growing on a rock. 
The cup-shaped structures are the fruiting bodies (apothecia). At the leftjare 
seen two very young plants. (Gager.) 
Lichens are found on the bark of trees, on rocks, logs, old fences, 
etc. The body of a lichen shows a differentiation into two regions: 
a more or less compact row of cells on both surfaces, called the epider- 
mis; and an inner portion composed of the mycelium of the fungus. 
The alga is imbedded in this portion. In most cases the'spores are 
borne in asci, which are themselves found in closed or open Apothecia. 
Scales or soredia are found on many lichens. These consist of a 
network of hyphae enclosing algal cells. By becoming detached 
from the parent plant, they develop new lichens and so constitute 
a means of vegetative propagation. 
