THE PTEKIDOPHYTES 



283 



261. The fern plant : the gametophyte and sexual reproduc- 

 tion. Upon moist earth, pots in greenhouses, etc., the asexual 

 fern spores germinate. First a stout green cell is produced, 

 and at its base there appears a hair-like cell, the first rhizoid 

 (Fig. 234, B and (7). The green cell grows rapidly and soon 

 divides to form a row of cells (Fig. 234, D). 



Thereafter the tip, by means of a special apical cell, expands 

 into a broad, heart-shaped plant (Figs. 234, E, and 235, A). 

 At the margin this plant is 

 one layer of cells in thick- 

 ness, but along the midrib 

 quite a cushion of cells is 

 formed. From the under- 

 side and near the base many 

 rhizoids grow. The presence 

 of chlorophyll, and contact 

 with moist surfaces neces- 

 sary for water supply, aid 

 the gametophyte in manu- 

 facturing its own food. 



262. The fern plant: sex- 

 ual reproduction. Antheridia 

 may be developed upon the 

 filamentous green cells be- 

 fore the heart-shaped gametophyte is formed, or upon the older 

 gametophyte they may develop toward the basal region and 

 on the underside (Fig. 235). The antheridium is a globular 

 structure with a single layer of wall cells (Fig. 236, A), and a 

 central cell 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 bryophytes. The sperm is, how- 

 ever, quite complex and seems well constructed for swimming 

 (Fig. 236, B). 



Also on the underside of the gametophyte and nearer the 

 apical region are the archegonia (Fig. 235). Only the necks 

 extend out from the surface, and these usually turn backward 



FIG. 235. J., a diagram of the underside 

 of a fully formed fern gametophyte, 

 showing rhizoids, antheridia, and ar- 

 chegonia ; #, a fern gametophyte from 

 which a young sporophyte is growing 



Both somewhat enlarged 



