FERNS. f§9 



structure of this spore-case quite carefully, since it will help 

 us to understand better than we otherwise could the remarkable 

 operations which it performs in scattering the spores. 



263. Structure of a sporangium. — If we examine one of the 

 sporangia in side vkw as shown in fig. 137, we note a promi- 

 nent row of cells which extend around the margin of the dorsal 

 edge from near the attachment of the stalk to the upper front 

 angle. The cells are prominent because of the thick inner 

 walls, and the thick radial walls which are perpendicular to the 

 inner walls. The walls on the back of this row and on its sides 

 are very thin and membranous. We should make this one 



Fig- '33- 

 Rhizome of sensitive fern (Onoclea sensibilis\ 



carefully, for the structure of these cells is especially adapted to 

 a special function which they perform. This row of cells is 

 termed the annulus, which means a little ring. While this is 

 not a complete ring, in some other ferns the ring is nearly com- 

 plete. 



264. The lip cells. — In the front of the sporangium is another 

 peculiar group of cells. Two of the longer ones resemble the 

 lips of some creature, and since the sporangium opens between 

 them they are sometimes termed the lip cells. These lip cells 



