DEVELOPMENT OF PLANTS 



235 



more pores at the top, and the least touch now causes the spores 

 to sift out in smoke-like puffs, hence the popular name of puff 

 balls. In the earth stars (Fig. 173) the outer layer of the 



Fig. 172. Diagram of a section of one of the puffballs, showing the 

 thick skin or periderm and the irregular cavities which are lined with 

 basidia. At the base the larger, sterile cavities of the stipe are shown. 



periderm splits into rather regular star-like segments or valves 

 which are hygroscopic. In damp weather these valves roll back, 

 in some species to such an extent as to lift the puff ball from the 



Fig. 173. One of the puff-balls popularly known as " earth stars," show- 

 ing the outer periderm splitting into star-like sections and the inner peri- 

 derm opening by a pore. 



ground, when it may be set rolling by the wind and thus bring 

 about a better discharge of the spores. 



98. Order e. Nidulariales or Birds Nest Fungi. — These mi- 

 nute and curious fungi may be found growing upon twigs or 

 upon bare ground in old fields, or upon dried dung (Fig. 174, A). 

 They differ chiefly from the puff balls in that the spore-bearing 

 cavities are surrounded by tougher hyphae. Consequently, when 



