O92 A TEXTBOOK OF GENERAL BOTANY 
the neck projects from the surface of the prothallus. Fertiliza- 
tion takes place when a spermatozoid swims from an anther- 
idium, enters an archegonium (Fig. 437), and fuses with the ego 

Fig. 437. Fertilization in a fern 
Above, spermatozoids are entering neck of archegonium ; lower left, vertical sec- 
tion through venter, showing a single spermatozoid in the nucleus of the ege ; 
lower right, horizontal section through venter, showing a later stage in the union 
of spermatozoid and egg nucleus. (Redrawn after Shaw) 
(Hig. 437). As in the case of the Bryales, water is necessary to 
accomplish fertilization, because the spermatozoid has no way of 
reaching the archegonium except by swimming. 
