PTERIDOPHYTA 389 
by means of its chloroplastids and absorbs water through its 
rhizoids. Usually a prothallus bears both archegonia and an- 
theridia (Fig. 432), but in some cases the antheridia and arche- 
gonia are borne on different prothalli. The antheridia (Figs. 482, 
433) are spherical and contain numerous spermatozoids. The 
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Fic. 432. Lower surface of prothallus, or gametophyte, of a fern 
Near the base of the prothallus are numerous rhizoids ; in the center are rounded 
antheridia containing spermatozoids; near the apex are archegonia which point 
toward the base 
spermatozoid (Fig. 433) 1s a spiral structure, toward one end 
of which are numerous cilia which enable it to swim through 
water. As inthe Bryales, an archegonium is a flask-shaped struc- 
ture. In the enlarged basal portion, or venter, there is a large 
egg (Fig. 434), and above this a smaller cell, the ventral-canal 
cell. In the neck of typical ferns there are two neck-canal 
cells. When the archegonium matures, the ventral-canal cell and 
