THE ORIGIN OF SEX IN PLANTS 25 



neither male nor female. It is neuter, bearing no 

 organs of sex. The spores, which are commonly borne 

 on the backs of the leaves in little brown capsules, 

 when they are shed upon the ground, germinate, and 

 produce each a small green scaly structure called a 

 prothallus. This is the sexual generation, and it bears 

 the organs of sex. The sexual cells, or gametes, pro- 

 duced by these differ widely in size and behaviour. The 



FIG. 13. 



Archegonia of Fern (Polypodium). A, is still closed; = the ovum or egg; 

 B, shows an Archegonium with the canal of the neck open, and ready for fertiliza- 

 tion. ( x 240. After Strasburger.) 



male gametes, or spermatozoids, are produced in large 

 numbers within the hemispherical antheridia (Fig. 12 (1-5) ), 

 from which they escape on rupture caused by swelling 

 in presence of water (Fig. 12 (6) ). Each is a spirally 

 coiled body, and shows active screw-like movements in 

 the water into which it escapes. The movements are due 

 to the lashing action of numerous fine cilia (Fig. 12 (8) ). 

 The form of this male gamete is different from those 

 in the Algae previously described. But in its small 

 size and active motility it resembles them ; and in 

 both cases the gamete is a naked living cell, including 

 a nucleus. The female gamete or egg of a Fern is also 



