24 HEREDITY AND EVOLUTION IN PLANTS 



moisture will cause the rupture of the antheridia, and the 

 sperms will be set free (Fig. 23). The mucilaginous matter 

 extruded from the archegonia contains a substance (malic 

 acid, in some ferns) which stimulates the sperms to swim 

 toward it. This they are enabled to do by the free 

 external water. On reaching the archegonia, they enter 

 it, and swim down the neck-canal to the egg. The sperm 

 that first reaches the egg penetrates it, and passes through 



FIG. 24. Fertilization in the fern, Onoclea. A, longitudinal section 

 of archegonium, showing the egg in the venter, and numerous sperms 

 passing down the neck-canal. B, an egg-cell in the venter. One sperm 

 has entered the nucleus, three sperms have failed to enter the egg. (After 

 W. R. Shaw.) 



its cytoplasm until it reaches the egg-nucleus, with which 

 it fuses, thus completing the act of fertilization (Fig. 24). 

 As soon as one sperm enters the egg-cell, the latter at once 

 forms a fertilization-membrane about itself, through which 

 the remaining sperms cannot enter. 



