262 Field, Forest, and Wayside Flowers 



round or watch-shaped box filled with spores (Fig. 

 72). The sporangia of many ferns are nearly sur- 

 rounded by an incomplete ring of large cells, whose 

 brownish walls are of a substance akin to cork. As 

 the sporangium grows older the 

 outer walls of these cells dry 

 and shrink; and as this shrink- 

 ing proceeds, the incomplete 

 ring begins to straighten itself 

 out. By so doing it pulls 

 upon the surrounding tissue 

 and ruptures the sporangium, 

 scattering the dust-like spores 

 to the four winds. The spo- 

 rangia of the great Osmundas 

 have no encompassing ring, but 



FIG. 72. Opening sporan- 

 gium of a Florida fern t h ey are sp }i t by the action of 

 (Pteris cretica). (Much 



magnified.) a little group of corky cells, 



which shrink together as they grow old, and thus 

 first strain and then rend the neighboring tissue. 



The spores of native outdoor-ferns remain dor- 

 mant through the winter and grow into prothalli 

 in the spring. 



The Hartford climbing-fern, the common sensi- 

 tive-fern (Fig. 73), and a few others have insti- 

 tuted a division of labor by which some fronds 



