LIFE HISTORY OF A FERN 25 



17. Nature of the Fertilized Egg. It will at once be 

 recognized that the fertilized egg, resulting from a union 

 with the sperm, possesses a double or diploid nature. 1 

 In recognition of its dual nature it is called the oosperm 

 (egg and sperm). 2 The oosperm, however, like the un- 



FIG. 25. Young embryo of a maidenhair fern (Adiantum concinnum}, 

 still surrounded by the archegonium, which has grown in size. L, leaf; 

 S, stem; R, root; F, foot. (After Atkinson.) 



fertilized egg, is still only one cell, though its nucleus com- 

 prises substances contributed by both egg and sperm. 

 In some cases the egg and sperm that unite in fertilization 

 may come from different parents; their fusion is then 

 called cross-fertilization. 



1 As distinguished from the unfertilized egg, which is of a single, or 

 Iiaploid nature. 



2 The term oospore is often used here, but this term lacks the advan- 

 tage of indicating the real nature of the fertilized egg. 



