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 rin 
Fic. 72.—Opening sporan- P 8 8) but 
Ct ae Mtoek they are split by the action of 
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 
