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 
Fee aren ae ae of chlorophyll and contact 
from the gametophyte (magnified about with moist surfaces aid it in 
three times) . . 
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, 
Fic. 212. Gametophyte of a fern 
