274 



INTRODUCTION TO BOTANY 



(fig. 215). Within the sporangium (fig. 210) many heavy- 

 walled asexual spores are produced. In most of the common 

 ferns the sporangia are of the form shown in figure 210. There 

 is a heavy ring of cells which extends over the wall of the 

 sporangium. When ripe these cells become dry, straighten out, 

 and tear the sporangium open ; the ring of cells then springs 

 forcibly back to its former position, and in doing so scatters 

 the spores. 



259. Sexual reproduction of a fern. The asexual fern spore 

 germinates upon moist earth, pots in greenhouses, etc. It soon 



grows into a broad, flat, 

 heart-shaped plant (fig. 212) 

 not at all like the fern plant 

 that we ordinarily see. This 

 plant is one layer of cells in 

 thickness at the margin, but 

 along the midrib a cushion 

 of several layers of cells is 

 formed. From the under- 

 side and near the base many 

 rhizoids grow. The presence 

 of chlorophyll and contact 

 with moist surfaces aid it in 

 manufacturing its own food. 

 Antheridia may be developed almost anywhere upon the 

 plant before it has become fully heart-shaped, and thereafter 

 they usually develop on the underside, toward the basal 

 region (fig. 212). The antheridium is a globular structure 

 with a single layer of wall cells (fig. 213) and a central 

 region in which usually thirty-two or sixty-four sperms are 

 formed. In size and number of cells this antheridium is much 

 simpler than that seen in the bryophytes. The sperm is, how- 

 ever, quite complex and seems well constructed for swimming. 

 On the underside of the plant and nearer the tip region 

 are the archegonia (figs. 212 and 214). Only the necks extend 

 outward from the surface, and these usually turn backward, 



FIG. 212. Gametophyte of a fern 



A, the gametophyte (magnified about ten 



times) ; B, young sporophyte growing 



from the gametophyte (magnified about 



three times) 



