306 



GENERAL BOTANY 



(Fig. 176, A, B, (7) and are covered in the maidenhair by the 

 reflexed margins of the pinnules (Fig. 168, b). Each sporan- 

 gium (Fig. 175) is composed of a stalk and an enlarged spore 

 case, or sporangium proper. This spore case is composed of an 

 outer layer of cells and an inner mass of cells which ultimately 

 form the spores. A mature sporangium is also furnished with 

 a ring of thick-walled cells, the annulus, which brings about the 

 final rupture of the sporangium and the scattering of the spores. 

 This process is effected as follows : The cells of the annulus are 

 filled with water during the early stages in the development 



FIG. 175. Sporangia of a fern, showing the development and 

 dissemination of spores 



a, sporangia containing young and mature spores ; b, c, opening of the sporangia at 

 the lip cells ; d, spore dissemination hy the movement of the annulus 



of the sporangium. As the sporangium matures and the spores 

 ripen, this water begins to evaporate, and the thin outer walls of 

 the annulus cells are pulled in and become U-shaped. They 

 are supposed to be pulled in by the cohesive strength of the 

 diminishing water in their cell cavities. As the outer walls be- 

 come concave the entire annulus bends backward and ruptures 

 the wall of the sporangium at its weakest point, called the stoma. 

 Finally, when the water is nearly withdrawn from the cells, its 

 tension breaks, and their overstretched, thick inner and radial 

 walls snap the annulus back to its original position (Fig. 175, d). 

 This snap throws the spores so that they can be disseminated 

 by wind and air currents. These spores are highly protected 

 cells which have the power of withstanding drought and other 



