250 DEVELOPMENT OF LICHENS 



of a white skin-like periderm which encloses a stipe and pileus 

 (Fig. 175, A, B). The basidia are formed in honeycomb-like 

 cavities on the outer surface of the pileus. As soon as the spores 

 are matured the stipe quickly elongates, rupturing the periderm 

 and lifting the pileus into the air (Fig. 175, C). In structure 

 and coloration, the Phallales are among the most attractive of the 

 Basidiomycetes, but the majority of the forms are regarded with 

 aversion, since the spore-bearing layer melts down into a slimy 

 mass which emits a carrion-like stench. 



Lichens 



100. The Nature of Lichens. — These remarkable plants are of 

 almost universal distribution upon tree trunks, rocks, old fences 

 and buildings, and upon the bare earth, where they form variously 

 colored incrustations or leaf-like branching bodies (Fig. 176). 



Fig. 176. Common species of Lichens: A, an erect branching form, 

 Cladonia — as, ascocarps. B, a foliaceous lichen, Sticta. C, Parmelia spread- 

 ing over the bark of tree. The centrally-placed ascocarps are surrounded 

 by smaller pycnidia. 



The lichen is one of the most remarkable plants in the vegetable 

 kingdom since it is a union of two separate plants, a fungus and 

 an alga. Naturally the relationship of the lichens to other groups 

 of plants has been a matter of dispute, some regarding them as 

 constituting an independent division, and by others they are 

 looked upon as fungi. The fact that the fungus forms the bulk 

 of the lichen and lives upon the alga largely as a parasite and is 

 usually alone capable of forming reproductive spore bodies would 



