THE FUNGI 



279 



of branches in warm, moist climates, where they are popularly 

 known as gray moss, since they resemble mosses very closely. 

 The grayish-green color of lichens is due to the fact that the 

 plant body is composed of two distinct plants : namely, a green 

 alga resembling Protococcus and a colorless fungus like the 

 molds. These two plants are associated as partners in the same 

 plant body, the alga furnishing the sugar made by photosynthesis 

 in its green cells, 

 and the fungus ab- 

 sorbing the water 

 and soil salts neces- 

 sary for the growth 

 of each partner. 

 The physical rela- 

 tions of the two 

 plants are such as 

 to enable them to 

 maintain their part- 

 nership to the best 

 advantage. The flat, 

 leaflike body of a 

 common foliaceous 

 lichen is composed 

 largely of molcllike 

 fungus hyphse, not 

 unlike a very dense 

 and regularly formed mycelium. The hyphse on the upper and 

 lower surfaces of the flat expansion are short and closely ad- 

 herent (Fig. 156, (7), thus forming the protective outer layers 

 termed respectively the upper and lower cortex. Between these 

 two layers the hyphse form a looser structure termed the medulla, 

 or central part of the lichen body. The algae are commonly dis- 

 tributed in a layer just beneath the upper cortex, where they 

 are most advantageously exposed to light. The fungus hyphse 

 either penetrate the algal cells by means of short branches 

 which absorb food from the vacuole, or they adhere closely to 

 the algse and absorb nutriment through their cell wall. 



FIG. 157. A common lichen, Parmelia, on the bark 

 of a hickory tree 



The spore-bearing cups are clearly shown 



