FIG. 156. Habit and structure of a common tree lichen, Physcia stellaris 



A, habit of plants bearing reproductive cups, or apothecia; B, section of a reproduc- 

 tive cup, showing the dark spore-bearing layer (hymenium) lining the cup ; C, struc- 

 ture of upper portion of plant body, including the green algal cells ; D, portion of the 

 hymenial layer, greatly magnified, showing spore sacs (asci), with dark-colored spores, 

 and the lighter paraphyses. From Bergen and Davis's " Principles of Botany " 



